Preimplantation genetic diagnosis for α-and β-double thalassemia

Development of preimplantation genetic testing for monogenic reference materials using next-generation sequencing

OBJECTIVE

Preimplantation genetic testing for monogenic disorders (PGT-M) has been used for over 20 years to detect many serious genetic conditions. However, there is still a lack of reference materials (RMs) to validate the test performance during the development and quality control of PGT-M.

METHOD

Sixteen thalassemia cell lines from four thalassemia families were selected to establish the RMs. Each family consisted of parents with heterozygous mutations for α- and/or β-thalassemia and two children, at least one of whom carried a homozygous thalassemia mutation (proband). The RM panel consisted of 12 DNA samples (parents and probands in 4 families) and 4 simulated embryos (cell lines constructed from blood samples from the four nonproband children). Four accredited genetics laboratories that offer verification of thalassemia samples were invited to evaluate the performance of the RM panel. Furthermore, the stability of the RMs was determined by testing after freeze‒thaw cycles and long-term storage.

RESULTS

PGT-M reference materials containing 12 genome DNA (gDNA) reference materials and 4 simulated embryo reference materials for thalassemia testing were successfully established. Next-generation sequencing was performed on the samples. The genotypes and haplotypes of all 16 PGT-M reference materials were concordant across the four labs, which used various testing workflows. These well-characterized PGT-M reference materials retained their stability even after 3 years of storage.

CONCLUSION

The establishment of PGT-M reference materials for thalassemia will help with the standardization and accuracy of PGT-M in clinical use.

Evaluating the application value of NGS-based PGT-A by screening cryopreserved MDA products of embryos from PGT-M cycles with known transfer outcomes

PURPOSE

To determine the application value of next-generation sequencing (NGS)-based preimplantation genetic testing for aneuploidies (PGT-A).

METHODS

We conducted a retrospective case-control study on a cohort of frozen-thawed embryo transfer (FET) cycles following preimplantation genetic testing for monogenic disorders (PGT-M) between 2014 and 2017. Cycles that produced live births or early miscarriages were divided into live birth group (n = 76) or miscarriage group (n = 19), respectively. The NGS-based aneuploidy screening was performed on the multiple displacement amplification (MDA) products of the embryonic trophectoderm biopsy samples that were cryopreserved following PGT-M.

RESULTS

In the live birth group, 75% (57/76) embryos were euploid and 14.5% (11/76) were aneuploid. The remaining 10.5% (8/76) embryos were NGS-classified mosaic with the high- (≥ 50%) and low-level (< 50%) mosaicism rates at 7.9% (6/76) and 2.6% (2/76), respectively. In the miscarriage group, only 23.5% (4/17) embryos were aneuploid, while 58.8% (10/17) were euploid and 17.6% (3/17) were NGS-classified mosaic with the high- and low-level mosaicism rates at 11.8% (2/17) and 5.9% (1/17), respectively. For live birth and miscarriage groups, the transferable rate was 82.9% (63/76) and 70.6% (12/17), respectively, whereas the untransferable rate was 17.1% (13/76) and 29.4% (5/17), respectively.

CONCLUSION

The application of NGS-based PGT-A remains questionable, as it may cause at least one in six embryos with reproductive potential to be discarded and prevent miscarriage in less than one in three embryos in single-gene disease carriers.

Long-read sequencing on the SMRT platform enables efficient haplotype linkage analysis in preimplantation genetic testing for β-thalassemia

PURPOSE

This study aimed to evaluate the value of long-read sequencing for preimplantation haplotype linkage analysis.

METHODS

The genetic material of the three β-thalassemia mutation carrier couples was sequenced using single-molecule real-time sequencing in the 7.7-kb region of the HBB gene and a 7.4-kb region that partially overlapped with it to detect the presence of 17 common HBB gene mutations in the Chinese population and the haplotypes formed by the continuous array of single-nucleotide polymorphisms linked to these mutations. By using the same method to analyze multiple displacement amplification products of embryos from three families and comparing the results with those of the parents, it could be revealed whether the embryos carry disease-causing mutations without the need for a proband.

RESULTS

The HBB gene mutations of the three couples were accurately detected, and the haplotype linked to the pathogenic site was successfully obtained without the need for a proband. A total of 68.75% (22/32) of embryos from the three families successfully underwent haplotype linkage analysis, and the results were consistent with the results of NGS-based mutation site detection.

CONCLUSION

This study supports long-read sequencing as a potential tool for preimplantation haplotype linkage analysis.

Comparison of chromosomal status in reserved multiple displacement amplification products of embryos that resulted in miscarriages or live births: a blinded, nonselection case-control study

Objective

To analyze chromosomal status in reserved multiple displacement amplification (MDA) products of embryos that result in miscarriages or live births.

Methods

Patients who underwent preimplantation genetic testing for monogenic disorders (PGT-Ms) without aneuploidy screening were included. The case group included 28 cycles that resulted in miscarriages. Controls included 56 cycles with live births. Comprehensive chromosomal screening (CCS) using next-generation sequencing (NGS) was performed on reserved MDA products from previous blastocyst trophectoderm biopsies. The incidence and type of chromosomal abnormalities in embryos resulting in miscarriages or live births were analyzed.

Results

Of 28 embryos resulting in miscarriages in the case group, the rate of chromosomal abnormalities was 53.6%, which was significantly greater than 14.3% for those resulting in live births in control group (P < 0.001). Whole-chromosome aneuploidy was not found in the control group but was noted in 25.0% of embryos in the case group. Although the rates of segmental abnormality and mosaicism were also greater in the case group, no significant differences were detected. One chaotic embryo in the control group progressed to live birth.

Conclusion

Chromosomal abnormalities were the main reason leading to early pregnancy loss. However, abnormalities, such as segmental aneuploidy and mosaicism, should be managed cautiously, considering their undermined reproductive potential.

Affected-embryo-based SNP haplotyping with NGS for the preimplantation genetic testing of Marfan syndrome

Marfan syndrome (MFS), an autosomal dominant heritable disease of the connective tissue, is characterized by broad clinical manifestations in the musculoskeletal, cardiovascular, pulmonary, and ocular systems. In this study, a male patient with MFS caused by a heterozygous mutation NM_000138.5(FBN1):c.6037 + 2 T > C in the fibrillin 1 gene (FBN1) underwent preimplantation genetic testing (PGT) by using affected-embryo-based single nucleotide polymorphism (SNP) haplotyping. Multiple displacement amplification was used for whole genome amplification of biopsied trophectoderm cells after controlled ovarian stimulation. Sanger sequencing and next-generation sequencing (NGS) were used to detect the state of FBN1 mutation. A total of 14 blastocysts formed after intracytoplasmic sperm injection were biopsied. After NGS, 60 informative polymorphic SNP markers located upstream and downstream of the FBN1 gene and its pathogenic mutation site were linked to individual alleles. Sanger sequencing further confirmed that 8 blastocysts carried the mutation NM_000138.5(FBN1):c.6037 + 2 T > C, while 6 did not. Four of the non-carriers were euploid verified by copy number variation results. A female infant without MFS was born at 37 weeks gestation after a subsequent frozen embryo transfer. In conclusion, the successful case indicates that SNP haplotyping using sibling embryos as a reference is applicable to PGT in monogenetic diseases.Abbreviations MFS: Marfan syndrome; PGT: preimplantation genetic testing; FBN1: fibrillin 1 gene; NGS: next-generation sequencing; SNP: single nucleotide polymorphism.

Successful four-factor preimplantation genetic testing: α- and β-thalassemia, human leukocyte antigen typing, and aneuploidy screening

Our study established an effective next-generation sequencing (NGS) protocol for four-factor preimplantation genetic testing (PGT) using  α- and β-thalassemia, human leukocyte antigen (HLA) typing, and aneuploidy screening. Three couples, in whom both partners were α- and β-double thalassemia carriers, underwent PGT between 2016 and 2018. These individuals sought an opportunity for hematopoietic stem cell transplantation to save their children from β-thalassemia major. A total of 35 biopsied trophectoderm samples underwent multiple displacement amplification (MDA). PGT for α- and β-thalassemia and HLA typing were performed on MDA products using NGS-based single-nucleotide polymorphism (SNP) haplotyping. Although two samples failed MDA, 94.3% (33/35) of samples were successfully amplified, achieving conclusive PGT results. Furthermore, 51.5% (17/33) of the embryos were diagnosed as unaffected non-carriers or carriers. Of the 17 unaffected embryos, nine (52.9%) were tested further  and identified as euploid via NGS-based aneuploid screening, in which five had HLA types matching affected children. One family did not achieve any unaffected euploid embryos. The two other families transferred HLA-matched and unaffected euploid embryos, resulting in two healthy 'savior babies.' NGS-PGT results were confirmed in prenatal diagnosis. Therefore, NGS-SNP was effective in performing PGT for multipurpose detection within a single PGT cycle.

Eleven healthy live births: a result of simultaneous preimplantation genetic testing of α- and β-double thalassemia and aneuploidy screening

PURPOSE

To evaluate the efficacy of preimplantation genetic testing (PGT) for α- and β-double thalassemia combined with aneuploidy screening using next-generation sequencing (NGS).

METHODS

An NGS-based PGT protocol was performed between 2017 and 2018 for twelve couples, each of which carried both α- and β-thalassemia mutations. Trophectoderm biopsy samples underwent whole-genome amplification using multiple displacement amplification (MDA), followed by NGS for thalassemia detection and aneuploidy screening. A selection of several informative single nucleotide polymorphisms (SNPs) established haplotypes. Aneuploidy screening was performed only on unaffected noncarriers and carriers. Unaffected and euploid embryos were transferred into the uterus through frozen-thawed embryo transfer (FET).

RESULTS

A total of 280 oocytes were retrieved following 18 ovum pick-up (OPU) cycles, with 182 normally fertilized and 112 cultured to become blastocysts. One hundred and seven (95.5%, 107/112) blastocysts received conclusive PGT results, showing 56 (52.3%, 56/107) were unaffected. Thirty-seven (66.1%, 37/56) of the unaffected were also identified as euploid. One family had no transferable embryos. Unaffected and euploid embryos were then transferred into the uterus of the other 11 couples resulting in 11 healthy live births. The clinical pregnancy rate was 61.1% (11/18) per OPU and 68.8% (11/16) per FET, with no miscarriage reported. Seven families accepted the prenatal diagnosis and received consistent results with the NGS-based PGT.

CONCLUSION

This study indicated that NGS could realize the simultaneous PGT of double thalassemia and aneuploidy screening in a reliable and accurate manner. Moreover, it eliminated the need for multiple biopsies, alleviating the potential damages to the pre-implanted blastocysts.

Multiple displacement amplification as the first step can increase the diagnostic efficiency of preimplantation genetic testing for monogenic disease for β-thalassemia

AIM

To evaluate whether using multiple displacement amplification (MDA) as the first step can increase the diagnostic efficiency of preimplantation genetic testing for monogenic disease (PGT-M) for β-thalassemia.

METHODS

This is a retrospective cohort study. All included patients underwent PGT-M cycles (n = 307) for β-thalassemia in our center from January 2014 to February 2018. We divided the patients into two groups based on two different detection methods. For the polymerase chain reaction (PCR) group (n = 115), multiplex nested PCR+ reverse dot blot analysis was performed directly after cell lysis. For the MDA group (n = 192), the whole genomes of single cells were directly amplified using MDA and then examined by singleplex PCR + reverse dot blot for β-thalassemia.

RESULTS

A total of 2315 embryos were tested. The overall diagnostic efficiency of the MDA group was significantly higher than that of the PCR group (96.99% vs 88.15%, P < 0.001). The percentage of embryos available for transfer was significantly higher in the MDA group than in the PCR group (74.28% vs 64.98%, P < 0.001). Furthermore, the carrier embryo rate of the MDA group was significantly higher than that of the PCR group (50.11% vs 35.95%, P < 0.001).

CONCLUSION

This study indicates that MDA, as the first step in PGT-M for β-thalassemia, can increase diagnostic efficiency.

Preimplantation Genetic Testing for Monogenic Disease of Spinal Muscular Atrophy by Multiple Displacement Amplification: 11 unaffected livebirths

Background: Preimplantation genetic testing for monogenic disease (PGT-M) has become an effective method for providing couples with the opportunity of a pregnancy with a baby free of spinal muscular atrophy (SMA). Multiple displacement amplification (MDA) overcomes the innate dilemma of very limited genetic material available for PGT-M. Objective: To evaluate the use of MDA, combined with haplotype analysis and mutation amplification, in PGT-M for families with SMA. Methods: MDA was used to amplify the whole genome from single blastomeres or trophectoderm (TE) cells. Exon 7 of the survival motor neuron gene 1 (SMN1) and eleven STRs markers flanking the SMN1 gene were incorporated into singleplex polymerase chain reaction (PCR) assays on MDA products. Results: Sixteen cycles (19 ovum pick-up cycles) of PGT-M were initiated in 12 couples. A total of 141 embryos were tested: 90 embryos were biopsied at the cleavage stage and 51 embryos were biopsied at the blastocyst stage. MDA was successful on 94.44% (85/90) of the single blastomeres and on 92.16% (47/51) of the TE cells. And the PCR efficiency were 98.4% (561/570) and 100% (182/182), respectively. In addition, the average allele drop-out (ADO) rates were 13.3% (60/392) and 9.8% (11/112), respectively. The results for SMN1 exon 7 were all matched with haplotype analysis, which allowed an accurate diagnosis of 93.62% (132/141) embryos. Twelve families had unaffected embryos available for transfer and a total of 38 embryos were transferred in 20 embryo transfer cycles. Eight transfers were successful, resulting in a clinical pregnancy rate of 40% (8/20) and an implantation rate of 28.95% (11/38). Finally, 11 healthy babies were born. Among them, 5 SMA carriers were singleton live births and 3 SMA carriers had twin births. Conclusion: Careful handling during the MDA procedure can improve subsequent PCR efficiency and reduce the ADO rate. We suggest that this protocol is reliable for increasing the accuracy of the PGT-M for SMA.

Genetic diagnosis of β-thalassemia preimplantation using short tandem repeats in human cryopreserved blastocysts

This study aimed to evaluate the application of short tandem repeats (STRs) for the preimplantation genetic diagnosis (PGD) of β-thalassemia. This was a prospective study performed at the Liuzhou Maternity and Child Healthcare Hospital. From May to December 2016, eight couples formed of two β-thalassemia carriers underwent in vitro fertilization (IVF) procedures and PGD. All couples and four family members/couple underwent blood testing. Whole genome amplification of trophectoderm cells was performed. PCR products were used for linkage analysis of 15 STR loci. From the eight couples, 147 embryos were obtained and 86 blastocysts were formed. The DNA from 82 blastocysts was successfully amplified (amplification efficiency of 95.4%). Eighty blastocysts obtained a definite diagnosis. Among them, 24 blastocysts were diagnosed as normal, 38 blastocysts were diagnosed as heterozygous for β-thalassemia, and 18 blastocysts were homozygous or compound heterozygous. Two patients received a thawed embryo and both had a clinical pregnancy. These results indicated that in the setting of PGD for β-thalassemia, after multiple displacement amplifications, reverse dot hybridization combined with STRs could be an effective, accurate, and practical clinical strategy to improve the detection of β-thalassemia in at-risk couples undergoing embryo transfer. These results have to be validated in a larger cohort.

Simple and Easy to Perform Preimplantation Genetic Diagnosis for β-thalassemia Major Using Combination of Conventional and Fluorescent Polymerase Chain Reaction

BACKGROUND

Thalassemias are the most common monogenic disorders in many countries throughout the world. The best practice to control the prevalence of the disease is prenatal diagnosis (PND) services. Extensive practicing of PND proved effective in reducing new cases but on the other side of this success high abortion rate is hided, which ethically unfair and for many couples, especially with a previous experience of a therapeutic abortion, or moral concerns, is not a suitable choice. Preimplantation genetic diagnosis (PGD) is a strong alternative to conventional PND. At present PGD is the only abortion free fetal diagnostic process. Considering the fact that there are more than 6000 single gene disorders affecting approximately 1 in 300 live-births, the medical need for PGD services is significant.

MATERIALS AND METHODS

In the present study development of a PGD protocol for a thalassemia trait couple using nested multiplex fluorescent polymerase chain reaction (PCR) for the combination of polymorphic linked short tandem repeat (STR) markers and thalassemia mutations is described. Restriction fragment length polymorphism used to discriminate between wild and mutated alleles.

RESULTS

In PGD clinical cycle, paternal and maternal alleles for D11S988 and D11S1338 STR markers were segregated as it was expected. PCR product for IVSII-1 mutation was subsequently digested with BtscI restriction enzyme to differentiate normal allele from the mutant allele. The mother's mutation, being a comparatively large deletion, was detectable through size differences on agarose gel.

CONCLUSION

The optimized single cell protocol developed and evaluated in this study is a feasible approach for preimplantation diagnosis of β-thalassemia in our patients.

Präimplantationsdiagnostik – methodische Aspekte

Die Präimplantationsdiagnostik erfordert eine enge und vertrauensvolle interdisziplinäre Zusammenarbeit zwischen hoch qualifizierten Fachärzten und Naturwissenschaftlern aus Humangenetik und Reproduktionsmedizin. In einem sehr engen Zeitfenster müssen komplexe Laborabläufe standardisiert und qualitätsgesichert umgesetzt werden. In diesem Beitrag sollen orientierende Empfehlungen zur Umsetzung kurz vorgestellt werden. Zentral haben wir häufigere Problemsituationen thematisiert, welche bereits bei der Indikationsstellung wie auch bei den nachfolgenden Schritten in der genetischen Analyse, Datenauswertung und Befunderstellung mögliche Fehlerquellen darstellen. Ziel unserer verantwortlichen Arbeit an den PID-Zentren sollte eine hohe Geburtenrate bei hoher Diagnosesicherheit mit möglichst wenigen schonenden Behandlungszyklen sein.

Endometrial thickness as a predictor of pregnancy outcomes in 10787 fresh IVF-ICSI cycles

This retrospective study assessed the predictive value of endometrial thickness (EMT) on HCG administration day for the clinical outcome of fresh IVF and intracytoplasmic sperm injection (ICSI) cycles. A total of 8690 consecutive women undergoing 10,787 cycles over a 5-year period were included. The 5th, 50th and 95th centiles for EMT were determined as 8, 11 and 15 mm, respectively. Group analysis according to these centiles (Group 1: < 8 mm; Group 2: ≥ 8 and ≤11 mm; Group 3: > 11 and ≤15 mm; Group 4: > 15 mm) demonstrated significant differences (P < 0.001) in clinical pregnancy rates (23.0%, 37.2%, 46.2% and 53.3%, respectively), live birth rates per clinical pregnancy (63.3%, 72.0%, 78.1% and 80.3%, respectively), spontaneous abortion rates (26.7%, 23.8%, 19.9% and 17.5%, respectively), and ectopic pregnancy rates (10.0%, 4.3%, 2.1% and 2.2%, respectively). Logistic regression analyses showed EMT as one of the independent variables predictive of clinical pregnancy (OR = 1.097; P < 0.001), live birth (OR = 1.078; P < 0.001), spontaneous abortion (OR = 0.948; P < 0.001), and ectopic pregnancy (OR = 0.851; P < 0.001). Future research should aim to understand the underlying mechanisms relating EMT to conception, ectopic implantation and spontaneous abortion.

Clinical Considerations of Preimplantation Genetic Diagnosis for Monogenic Diseases

Purpose

The aim of this study was to explore factors contribute to the success of PGD cycles for monogenic diseases.

Methods

During a 3-year period (January 2009 to December 2012), 184 consecutive ICSI-PGD cycles for monogenic diseases reaching the ovum pick-up and fresh embryo-transfer stage performed at the Reproductive Medicine Center of The First Affiliated Hospital Of Sun Yat-sen University were evaluated.

Results

ICSI was performed on 2206 metaphase II oocytes, and normal fertilization and cleavage rates were 83.4% (1840/2206) and 96.2% (1770/1840), respectively. In the present study, 60.5% (181/299) of day 3 good-quality embryos developed into good-quality embryos on day 4 after biopsy. Collectively, 42.9% clinical pregnancy rate (79/184) and 28.5% implantation rate (111/389) were presented. In the adjusted linear regression model, the only two significant factors affecting the number of genetically unaffected embryos were the number of biopsied embryos (coefficient: 0.390, 95%CI 0.317–0.463, P = 0.000) and basal FSH level (coefficient: 0.198, 95%CI 0.031–0.365, P = 0.021). In the adjusted binary logistic regression model, the only two significant factors affecting pregnancy outcome were the number of genetically available transferable embryos after PGD (adjusted OR 1.345, 95% CI 1.148–1.575, P = 0.000) and number of oocyte retrieved (adjusted OR 0.934, 95% CI 0.877–0.994, P = 0.031).

Conclusion

There should be at least four biopsied embryos to obtain at least one unaffected embryos in a PGD system for patients with single gene disorder and under the condition of basal FSH level smaller than 8.0mmol/L. Moreover, if only a low number (< 4) of biopsied embryos are available on day 3, the chance of unaffected embryos for transfer was small, with poor outcome.

Medium-based noninvasive preimplantation genetic diagnosis for human α-thalassemias-SEA

To develop a noninvasive medium-based preimplantation genetic diagnosis (PGD) test for α-thalassemias-SEA. The embryos of α-thalassemia-SEA carriers undergoing in vitro fertilization (IVF) were cultured. Single cells were biopsied from blastomeres and subjected to fluorescent gap polymerase chain reaction (PCR) analysis; the spent culture media that contained embryo genomic DNA and corresponding blastocysts as verification were subjected to quantitative-PCR (Q-PCR) detection of α-thalassemia-SEA. The diagnosis efficiency and allele dropout (ADO) ratio were calculated, and the cell-free DNA concentration was quantitatively assessed in the culture medium. The diagnosis efficiency of medium-based α-thalassemias-SEA detection significantly increased compared with that of biopsy-based fluorescent gap PCR analysis (88.6% vs 82.1%, P < 0.05). There is no significant difference regarding ADO ratio between them. The optimal time for medium-based α-thalassemias-SEA detection is Day 5 (D5) following IVF. Medium-based α-thalassemias-SEA detection could represent a novel, quick, and noninvasive approach for carriers to undergo IVF and PGD.

The prevalence of chromosomal deletions relating to developmental delay and/or intellectual disability in human euploid blastocysts

Chromosomal anomalies in human embryos produced by in vitro fertilization are very common, which include numerical (aneuploidy) and structural (deletion, duplication or others) anomalies. Our previous study indicated that chromosomal deletion(s) is the most common structural anomaly accounting for approximately 8% of euploid blastocysts. It is still unknown if these deletions in human euploid blastocysts have clinical significance. In this study, we analyzed 15 previously diagnosed euploid blastocysts that had chromosomal deletion(s) using Agilent oligonucleotide DNA microarray platform and localized the gene location in each deletion. Then, we used OMIM gene map and phenotype database to investigate if these deletions are related with some important genes that cause genetic diseases, especially developmental delay or intellectual disability. As results, we found that the detectable chromosomal deletion size with Agilent microarray is above 2.38 Mb, while the deletions observed in human blastocysts are between 11.6 to 103 Mb. With OMIM gene map and phenotype database information, we found that deletions can result in loss of 81-464 genes. Out of these genes, 34–149 genes are related with known genetic problems. Furthermore, we found that 5 out of 15 samples lost genes in the deleted region, which were related to developmental delay and/or intellectual disability. In conclusion, our data indicates that all human euploid blastocysts with chromosomal deletion(s) are abnormal and transfer of these embryos may cause birth defects and/or developmental and intellectual disabilities. Therefore, the embryos with chromosomal deletion revealed by DNA microarray should not be transferred to the patients, or further gene map and/or phenotype seeking is necessary before making a final decision.

Preimplantation testing: Transition from genetic to genomic diagnosis

Preimplantation genetic testing refers to the procedure to determine the genetic status of embryos formed by in vitro fertilization (IVF) prior to initiating a pregnancy. Traditional genetic methods for preimplantation genetic diagnosis (PGD) examine distinct parts of an individual genome, require the development of a custom assay for every patient family, and are time consuming and inefficient. In the last decade technologies for whole-genome amplification (WGA) from single cells have led to innovative strategies for preimplantation testing. Applications of WGA technology can lead to a universal approach that uses single-nucleotide polymorphisms (SNPs) and mutations across the entire genome for the analysis. Single-cell WGA by multiple displacement amplification has enabled a linkage approach to PGD known as “preimplantation genetic haplotyping”, as well as microarray-based techniques for preimplantation diagnosis. The use of microarrays in preimplantation diagnosis has provided genome-wide testing for gains or losses of single chromosomes (aneuploidies) or chromosomal segments. Properly designed randomized controlled trials are, however, needed to determine whether these new technologies improve IVF outcomes by increasing implantation rates and decreasing miscarriage rates. In genotype analysis of single cells, allele dropout occurs frequently at heterozygous loci. Preimplantation testing of multiple cells biopsied from blastocysts, however, can reduce allele dropout rates and increase the accuracy of genotyping, but it allows less time for PGD. Future development of fast SNP microarrays will enable a universal preimplantation testing for aneuploidies, single-gene disorders and unbalanced translocations within the time frame of an IVF cycle.