New insights into the prognostic value of Ki-67 labeling index in patients with triple-negative breast cancer

Is Programmed Death-Ligand 1 of Prognostic Significance in Triple-Negative Female Mammary Carcinoma?

Introduction

The most widespread female malignancy is breast cancer (BC), considerable percentage of patients with triple-negative BC (TNBC) experience rapid progression, recurrence, and metastasis. BC has not historically been treated as an immunogenic cancer. Nonetheless, several researchers have started to concentrate on immunotherapy.

Aim

The aim of the study is to investigate the immunohistochemical (IHC) expression of programmed death-ligand 1 (PD-L1) by stromal tumor-infiltrating lymphocytes (TILs) and tumor cells (TC) in female (TNBC) and to correlate with pathological features of such tumors, particularly those determine biologic behavior, such as the grade and stage the overall survival.

Methodology

This is a retrospective study which includes 49 paraffin-embedded tumor tissue sections which were collected from breast surgery specimens either radical or conservative of female patients with TNBC. The samples were analyzed immunohistochemically for PD-L1 expression.

Results

There were statistically significant relations among TC PD-L1 expression and TILs PD-L1 expression as well as relations among TILs PD-L1 expression with histologic grade, stromal TILs, and Ki-67 were statistically significant. Correlations between TC PD-L1 expression and N stage, histologic grade, and anatomic stage were statistically significant. Improved survival was detected within TILs PD-L1-positive cases; however, the correlation between the overall survival and PD-L1 expression in both TCs and stromal TIL was not statistically significant.

Conclusion

PD-L1 expressed in tumors with poor prognostic features such as the high grade, advanced T stage, and high Ki-67 index, TILs PD-L1-positive cases experienced improved survival supporting its prognostic significance. However, the correlation with overall survival was not statistically significant.

Does a 40% Cut-off Point for Ki-67 Expression Have a Role in Identifying the Development of Distant Metastasis Within 2 Years in Locally Advanced Triple Negative Breast Cancer Patients?

Objective

Triple negative breast cancer (TNBC) has a higher proportion of patients with distant recurrence or metastasis. Ki-67 has been suggested as an essential factor in cancer grading and prognostic evaluation, although there is still a debate regarding the Ki-67 cut-off value in TNBC. The aim of this study was to determine the role of Ki-67 expression using a 40% cut-off point as a risk factor for developing distant metastasis within two years in patients with TNBC.

Materials and Methods

This analytical observational study was conducted with a case-control design from January 2021-2022. Subjects were divided into two groups (metastasis within two years or more than two years after diagnosis). Bivariate analysis was conducted using chi-square test and odds ratio (OR) was also analyzed.

Results

A total of 66 subjects were included. In patients with metastasized TNBC and a Ki-67 expression of ≥40%, 29 patients (55.8%) had metastasis occurring in ≤2 years and 23 patients (44.2%) had metastasis occurring in >2 years; in patients with metastasized TNBC and a Ki-67 expression of <40%, 4 patients (28.6%) had metastasis occurring in ≤2 years and 10 patients (71.4%) had metastasis occurring in >2 years. Chi-square analysis (p = 0.071) indicated no significant association between patients with Ki-67 expression of ≥40% and <40% with metastasis within 2 years [OR 3.152 (confidence interval: 95% 0.875-11.362)].

Conclusion

Ki-67 protein expression of over 40% in patients with locally-advanced TNBC does not indicate a greater risk of distant metastasis in the first two years after diagnosis.

In silico recognition of a prognostic signature in basal-like breast cancer patients

Background

Triple-negative breast cancers (TNBCs) display poor prognosis, have a high risk of tumour recurrence, and exhibit high resistance to drug treatments. Based on their gene expression profiles, the majority of TNBCs are classified as basal-like breast cancers. Currently, there are not available widely-accepted prognostic markers to predict outcomes in basal-like subtype, so the selection of new prognostic indicators for this BC phenotype represents an unmet clinical challenge.

Results

Here, we attempted to address this challenging issue by exploiting a bioinformatics pipeline able to integrate transcriptomic, genomic, epigenomic, and clinical data freely accessible from public repositories. This pipeline starts from the application of the well-established network-based SWIM methodology on the transcriptomic data to unveil important (switch) genes in relation with a complex disease of interest. Then, survival and linear regression analyses are performed to associate the gene expression profiles of the switch genes with both the patients’ clinical outcome and the disease aggressiveness. This allows us to identify a prognostic gene signature that in turn is fed to the last step of the pipeline consisting of an analysis at DNA level, to investigate whether variations in the expression of identified prognostic switch genes could be related to genetic (copy number variations) or epigenetic (DNA methylation differences) alterations in their gene loci, or to the activities of transcription factors binding to their promoter regions. Finally, changes in the protein expression levels corresponding to the so far identified prognostic switch genes are evaluated by immunohistochemical staining results taking advantage of the Human Protein Atlas.

Conclusion

The application of the proposed pipeline on the dataset of The Cancer Genome Atlas (TCGA)-Breast Invasive Carcinoma (BRCA) patients affected by basal-like subtype led to an in silico recognition of a basal-like specific gene signature composed of 11 potential prognostic biomarkers to be further investigated.

Associations of Estrogen Receptor, Progesterone Receptor, Human Epidemic Growth Factor Receptor-2 and Ki-67 with Ultrasound Signs and Prognosis of Breast Cancer Patients

Objective

The functions of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2) and Ki-67 in breast cancer have been explored. This study was carried out to explore ER, PR, HER-2 and Ki-67 expression levels in breast cancer patients and their relationship with ultrasound signs and prognosis.

Patients and Methods

A total of 274 female primary breast cancer patients received preoperative ultrasound examination. ER, PR, HER-2 and Ki-67 expression levels in breast cancer tissues were detected by immunohistochemical staining after surgery. The correlations of ER, PR, HER-2 and Ki-67 expression with ultrasound signs and prognosis of breast cancer patients were analyzed.

Results

The positive expression rate of ER, PR and HER-2 and Ki-67 high expression in 274 breast cancer patients was 73.36% (201/274), 59.85% (164/274), 24.09% (66/274) and 66.06% (181/274), respectively. ER-positive expression had association with lymph node metastasis (LNM) and blood flow grading; HER-2-positive expression was associated with LNM, while Ki-67-positive expression was related to the tumor diameter, LNM, and blood flow grading. LNM and Ki-67 high expression were risk factors for OS; PR-positive was a protective factor for OS; TNM stage, tumor diameter, LNM and Ki-67 high expression were risk factors for DFS in breast cancer patients.

Conclusion

ER, PR, HER-2 and Ki-67 in breast cancer are related to the ultrasound signs and prognosis of breast cancer patients. The joint detection of multiple indicators provides a reference for the individualized treatment of targeted drugs.

KI-67 LI Expression in Triple-Negative Breast Cancer Patients and Its Significance

Purpose:

Triple-negative breast cancer (TNBC) is a subset of breast cancer which is known to carry a poor prognosis because of lack of targets for hormonal therapy. Research efforts have focused in recent years on discovering biomarkers of management in TNBCs. KI-67 Labelling Index (LI) is a nuclear protein which has proven to play diagnostic and prognostic roles in many cancers.

Materials and methods:

We analysed the expression of KI-67 LI by immunohistochemistry in TNBC cases from the University hospital. This expression was cross-checked against clinical-pathological criteria of TNBC patients and against Vimentin expression in TNBC patients with significant KI-67 expression.

Results:

KI-67 LI was significantly expressed in the majority of TNBC cases. This expression was significantly correlated with lymph node metastases, tumour invasion, high tumour nuclear grade, clinical stage, adverse survival outcome, and failure to achieve pathological complete response. TNBCs’ KI-67 LI expression was also correlated with Vimentin expression, the mesenchymal chief marker of the EMT phenomenon.

Conclusion:

Collectively, our study presents a strong argument for the use of KI-67 LI as a biomarker of aggressive, metastatic TNBC disease with poor outcome. This study, along with mounting evidence in the scientific literature, presents a case for the use of this nuclear protein in diagnosis, prognosis, and follow-up of patients with this difficult diagnosis.

Prognostic Value of Ki-67 in Patients With Resected Triple-Negative Breast Cancer: A Meta-Analysis

Background: Ki-67 is a widely used marker of tumor proliferation, but the prognostic value of ki-67 in triple-negative breast cancer (TNBC) has not been comprehensively reviewed. This meta-analysis was conducted to evaluate the association between ki-67 expression and survival of patients with resected TNBC.

Materials and Methods: Relevant studies, evaluating the prognostic impact of pretreatment ki-67 in resected TNBC patients, were identified from PubMed, Embase, Web of Science, China National Knowledge Infrastructure, and Cochrane Library until March 14, 2019. Hazard ratios (HRs) with 95% confidence intervals (CI) were calculated as effect values for disease-free survival (DFS) and overall survival (OS).

Results: In present meta-analysis, 35 studies with 7,716 enrolled patients were eligible for inclusion. Pooled results showed that a high ki-67 expression was significantly associated with poor DFS (HR = 1.73, 95% CI: 1.45–2.07, p < 0.001) and poor OS (HR = 1.65, 95% CI: 1.27–2.14, p < 0.001) in resected TNBC. In the subgroup analysis, when a cutoff of Ki-67 staining ≥40% was applied, the pooled HR for DFS and OS was 2.30 (95% CI 1.54–3.44, p < 0.001) and 2.95 (95% CI 1.67–5.19, p < 0.001), respectively.

Conclusion: A high Ki-67 expression is a poor prognostic factor of resected TNBC. The cut-off of ki-67 ≥40% is associated with a greater risk of recurrence and death compared with lower expression rates, despite the Ki-67 threshold with the greatest prognostic significance is as yet unknown.

The role of Ki-67 in Asian triple negative breast cancers: a novel combinatory panel approach

The proliferation marker Ki-67 is frequently used to assess aggressiveness in the pathological evaluation of cancer, but its role remains uncertain in triple-negative breast cancer (TNBC). We aimed to quantify and localize Ki-67 expression in both epithelial and immune compartments in TNBC and investigate its association with clinicopathological parameters and survival outcomes. A total of 406 TNBC cases diagnosed between 2003 and 2015 at Singapore General Hospital were recruited. Using state-of-the-art, 7-colour multiplex immunofluorescence (mIF) tissue microarrays (TMAs) were stained to assess the abundance, density and spatial distribution of Ki-67-positive tumour cells and immune cells co-decorated with cytokeratin (CK) and leukocyte common antigen (CD45) respectively. Furthermore, MKI67 mRNA profiles were analysed using NanoString technology. In multivariate analysis adjusted for tumour size, histologic grade, age at diagnosis, and lymph node stage, a high Ki-67 labelling index (LI) > 0.3% was associated with improved disease-free survival (DFS; HR = 0.727; p = 0.027). High Ki-67-positive immune cell count per TMA was a favourable prognostic marker for both DFS (HR = 0.379; p = 0.00153) and overall survival (OS; HR = 0.473; p = 0.0482). The combination of high Ki-67 LI and high MKI67 expression was associated with improved DFS (HR = 0.239; p = 0.00639) and OS (HR = 0.213; p = 0.034). This study is among the first to highlight that Ki-67 is associated with favourable prognosis in an adjuvant setting in TNBC, and the mIF-based evaluation of Ki-67 expression on both tumour and immune cells represents a novel prognostic approach.

Combined Detection of HER2, Ki67, and GSTP1 Genes on the Diagnosis and Prognosis of Breast Cancer

OBJECTIVE

Breast cancer (BC) is a common malignant tumor in females. The combined assay of multiple molecular markers benefits the diagnosis and prognostic prediction. Human epidermal growth factor receptor 2 (HER2) facilitates the proliferation and differentiation of cancer cells through ligand binding. Ki67 is a tumor proliferation-related gene, whereas GSTP1 is a DNA repair-related gene. This study thus investigated the significance of HER2 and Ki67/GSTP1 gene combined assay in the diagnosis and prognosis of BC.

MATERIALS AND METHODS

A total of 86 breast tumor tissues and adjacent tissues were collected. Gene expression and protein levels of HER2 and Ki67 were quantified by real-time polymerase chain reaction (PCR) and Western blot, respectively. Methylation frequency of GSTP1 was analyzed by methylation-specific PCR. The correlation between HER2 and Ki67/GSTP1 and clinical/pathological features of BC was analyzed.

RESULTS

Gene and protein expression levels of HER2 and Ki67 in tumor tissues were increased (p < 0.05 compared with adjacent tissues). Methylation frequency of GSTP1 gene was 37.2%, which was significantly higher in breast tumor tissues than in adjacent tissues (12.79%, p < 0.05). HER2 expression was positively correlated with TNM stage, tumor size, and lymph node metastasis, and negatively correlated with tissue grade and estrogen receptor (ER)/progesterone receptor (PR) expression (p < 0.05). GSTP1 methylation was positively correlated with TNM stage and tumor size, and negatively correlated with ER/PR expression (p < 0.05).

CONCLUSIONS

HER2, Ki67, and GSTP1 methylation were correlated with clinical and pathological features of BC. The combined assay benefits the early diagnosis and prognostic prediction of cancer.

Topoisomerase II α Gene alteration in Triple Negative Breast Cancer and Its Predictive Role for Anthracycline-Based Chemotherapy (Egyptian NCI Patients)

Objective:

Triple negative breast cancer is an aggressive variant of breast cancer; it forms about 15% of breast cancer cases. It lacks the responsiveness to hormonal and targeted therapies. Anthracyclines remain the treatment option for these patients. Anthracyclines are cardiotoxic, so predicting sensitivity of response by biological predictors may have a role in selecting suitable candidates for these drugs.

Material and methods:

This study included 50 TNBC cases, from National Cancer Institute, Cairo University(NCI-CU), Egypt, who underwent surgery and received adjuvant chemotherapy. Archived blocks were obtained and immunostaining for Ki-67 LI and Fluorescent In situ Hybridization (FISH) technique to assess TOP2A gene copy number and chromosome 17CEP status were done. Analysis of association between TOP2A alterations and CEP17 polysomy as well as Ki-67 LI with other clinicopathological parameters was done. Associations between the biological markers and event free survival (EFS) and overall survival (OS), were also performed.

Results:

TOP2A alteration was seen in 9/50 cases (5 amplified and 4 deleted). CEP17 Polysomy was detected in 14% of cases. Most of patients (80%) showed Ki-67 LI ≥20%. There was a significant association between TOP2A gene and CEP17 status. Outcome was better with abnormal TOP2A gene status and CEP17 polysomy, radiotherapy and combined anthracyclines and taxanes in the adjuvant setting, however P-values were not significant.

Conclusion:

TOP2A gene alterations and CEP17 polysomy may have prognostic and predictive role in TNBC treated with adjuvant Anthracyclines.

KLP-PI: a new prognostic index for luminal B HER-2-negative breast cancer

Luminal B HER-2-negative (LBHN) subtype is one of the major subtypes of breast cancer according to different features, clinical behaviors, and treatment response. The LBHN subtype shows a poor prognosis and is insensitive to endocrine therapy. Our work aim is to investigate the prognostic factor in the LBHN subgroup and, meanwhile, try to obtain an optimal prognostic index (PI) contrapose LBHN subgroup which helps to guide chemotherapy. A total of 515 female LBNH patients who underwent diagnosis and surgery at our hospitals from August 2008 to August 2018 were enrolled. Clinical-pathological information was obtained and immunohistochemistry result was available. From these cases, a 30% Ki-67 LI was employed to divide LBHN into two groups with low and high levels; high Ki-67 LI was associated with GIII tumor grade (P < 0.001), positive axillary lymph nodes (ALN) status (P = 0.018) and negative PR status (P = 0.016), and also seemed to be related to T2-T3 tumor size (P = 0.058). High Ki-67 level (HR = 3.30; P < 0.011), positive ALN (HR = 7.29; P < 0.001) and PR negative (HR = 2.63; P = 0.034) significantly associated with poor 5-year DFS in multivariate Cox's proportional hazard regression model. A novel prognosis prediction model (KLP-PI), based on Ki-67 LI, ALN and PR status, showed a better discriminatory ability compared with traditional Nottingham prognostic index targeted to LBHN breast cancer. Our study highlights that high Ki-67 LI, positive ALN and negative PR status were associated with poor outcome in LBHN patients, and composed by these prognostic factors, KLP-PI improves the prognostic assessment using the Nottingham Prognostic Index when aiming at LBHN subtype.

Expression and Clinical Significance of Claudin-7, PDL-1, PTEN, c-Kit, c-Met, c-Myc, ALK, CK5/6, CK17, p53, EGFR, Ki67, p63 in Triple-negative Breast Cancer-A Single Centre Prospective Observational Study

BACKGROUND/AIM

To explore the relationship between p53, p63, c-kit, Ki67, cMet, claudin7, CK5/6, CK17, AR, PTEN, EGFR, ALK, PDL-1 and c-MYC expression with the clinicopathological features of triple- negative breast cancer.

MATERIALS AND METHODS

Immunohistochemistry was performed in 84 triple-negative breast cancer samples.

RESULTS

A statistically significant relationship between tumour grade and claudin-7 (p=0.004) and between protein p53 and positive lymph nodes (p=0.015) was found. High expression of claudin-7 (OR=65.8, 95%CI=4.35-995.19, p-value=0.003) and low expression of c-kit (OR=0.14, 95%CI=0.025-0.793, p-value=0.026) and protein p63 (OR=0.18 95%CI=0.035-0.978, p-value=0.047) was associated with higher tumour grade. Higher AR expression (OR=13.44, 95%CI=1.28-141.56, p-value=0.031) and lower expression of CK5/6 cytokeratins was found in patients with positive lymphovascular invasion (LVI) (OR=0.072, 95%CI=0.007-0.732, p-value=0.026). Only the cell proliferation index (Ki67) has been proven to be statistically significant for disease-free survival (p-value=0.0378), and overall survival (p-value=0.0186).

CONCLUSION

High expression of claudin-7 and low expression of c-kit and protein p63 are associated with higher tumour grade. AR and CK5/6 expression seem to be important in LVI.

Prognostic value of Ki-67 according to age in patients with triple-negative breast cancer

PURPOSE

The prognostic value of Ki-67 in triple-negative breast cancer (TNBC) is yet unclear because the cut-off points employed differ widely and its predictive effect may vary according to age. The purpose of this study was to analyze the role of Ki-67 among patients with TNBC, and determine the optimal Ki-67 cut-off point to demonstrate its prognostic relevance associated with patient age and treatment strategy.

METHODS/PATIENTS

201 consecutive patients treated for primary TNBC from 1999 to 2014 were analyzed. Clinicopathological characteristics and outcomes were compared between patients treated with neoadjuvant or adjuvant chemotherapy. We used time-dependent receiver operating characteristic (ROC) curve and time-dependent area under the ROC curve (AUC) to evaluate the discriminative ability of Ki-67 at 3 and 5 years of follow-up. A Ki-67 cut-off point that maximized sensibility and specificity was established. Interaction effect between age and Ki-67 on disease-free survival (DFS) and overall survival (OS) was evaluated by stratified analysis.

RESULTS

According to the coordinates of the ROC curves, the best cut-off point for Ki-67 was 60% (high/low). In the whole group, there was not a statistically significant association between Ki-67 and OS and DFS, using a cut-off point of 60%. In multivariate analysis (COX proportional hazards regression), for DFS high Ki-67 (> 60%) was a poor prognostic factor in patients > 40 years old and a better prognostic factor among the patients < 40 years old.

CONCLUSION

Prognostic value of Ki-67 in TNBC, using a cut-off point of 60%, may vary depending on age.

Short hairpin RNA-mediated gene silencing of ADAM17 inhibits the growth of breast cancer MCF‑7 cells in vitro and in vivo and its mechanism of action

A disintegrin and metalloprotease 17 (ADAM17) is highly expressed in many malignant tumors and is closely related to their development. We showed in a previous study that silencing of ADAM17 by siRNA inhibited the growth of MCF-7 breast cancer cells in vitro and in vivo. In the present study, we investigated the effects of ADAM17-short hairpin RNA (ADAM17-shRNA) on MCF-7 breast cancer cells and explored the potential action pathway. In vitro, transfection of shRNAs was performed using a lentivirus, and the effects of ADAM17-shRNA on invasion, proliferation and cell cycle distribution of MCF-7 cells were assessed by Boyden chamber method, real-time cell analysis and flow cytometry, respectively. In vivo, MCF-7 cells with different administrations were transplanted subcutaneously into nude mice, and the effect of ADAM17-shRNA on the growth of transplanted tumors was assessed. In addition, the morphological structures were observed by H&E staining, and the expression of ADAM17 and Ki-67 was assessed by immunohistochemistry; expression of ADAM17, EGFR, p-EGFR, AKT, p-AKT, ERK and p-ERK proteins was assessed by western blotting, respectively. Our data showed that ADAM17-shRNA successfully inhibited ADAM17 mRNA expression, invasion and proliferation of MCF-7 cells resulting in G0/G1 phase arrest, and significantly inhibited the growth of transplanted tumors with larger areas of necrosis, low expression of ADAM17 and Ki-67 and reduced protein expression of ADAM17, EGFR, p-EGFR, AKT, p-AKT, ERK, and p-ERK in the tumor tissues. The present research suggests that ADAM17-shRNA can inhibit MCF-7 cell invasion and proliferation in vitro and inhibit MCF-7 xenograft growth in vivo through the EGFR/PI3K/AKT and EGFR/MEK/ERK signaling pathways.

The diagnostic and prognostic values of Ki-67/MIB-1 expression in thyroid cancer: a meta-analysis with 6,051 cases

Background

Growing evidence has demonstrated that Ki-67/MIB-1 has an effect on the clinical progression and prognosis in cancers. However, the diagnostic and prognostic values of Ki-67/MIB-1 in thyroid cancer remain unclear.

Materials and methods

The meta-analysis was conducted in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Studies were retrieved from PubMed, EBSCO, EMBASE, ISI Web of Science, China National Knowledge Infrastructure, WanFang and Chinese VIP databases. MetaDiSc and STATA12.0 were used to analyze the meta-analysis. Fixed-effect analysis and random-effect analysis were applied to pool the relative ratio based on heterogeneity in this meta-analysis.

Results

In the meta-analysis, 51 eligible studies were included. The pooled sensitivity of Ki-67/MIB-1 was 0.61 (95% confidence interval [CI]: 0.59–0.63) and specificity was 0.75 (95% CI: 0.74–0.77) in thyroid cancer. The pooled positive likelihood ratio was 3.19 (95% CI: 2.30–4.42) and negative likelihood ratio was 0.43 (95% CI: 0.35–0.54). In the diagnosis of thyroid cancer, the pooled diagnostic odds ratio of Ki-67/MIB-1 was 8.54 (95% CI: 5.03–14.49). The area under the symmetric receiver operating characteristic curve was 0.804 (standard error =0.031). Our results showed that there were statistical associations between Ki-67/MIB-1 and age (odds ratio [OR] =1.71, 95% CI: 1.14–2.57, P=0.010), tumor size (OR =1.86, 95% CI: 1.17–2.96, P=0.008), lymph node metastasis (OR =2.49, 95% CI: 1.42–4.39, P=0.002), metastasis status (OR =6.96, 95% CI: 2.46–19.69, P<0.001), tumor node metastasis stage (OR =6.56, 95% CI: 3.80–11.34, P<0.001) and extrathyroid extension (OR =1.91, 95% CI: 1.27–2.87, P=0.002). Furthermore, thyroid cancer patients with a high level of Ki-67/MIB-1 had a worse disease-free survival as compared to patients with a low level of Ki-67/MIB-1 (hazard ratio =5.19, 95% CI: 3.18–8.46, P<0.001). Also, Ki-67/MIB-1 was found to be associated with increased risk of mortality (hazard ratio =3.56, 95% CI: 1.17–10.83, P=0.025).

Conclusion

Our results demonstrated that Ki-67/MIB-1 might act as a potential factor in diagnosing thyroid cancer in Chinese. Also, the meta-analysis indicated that Ki-67/MIB-1 might have an effect on prognosis in non-Chinese thyroid cancer patients.

Cell-Cycle Regulation Accounts for Variability in Ki-67 Expression Levels

The cell proliferation antigen Ki-67 is widely used in cancer histopathology, but estimations of Ki-67 expression levels are inconsistent and understanding of its regulation is limited. Here we show that cell-cycle regulation underlies variable Ki-67 expression in all situations analyzed, including nontransformed human cells, normal mouse intestinal epithelia and adenomas, human cancer cell lines with or without drug treatments, and human breast and colon cancers. In normal cells, Ki-67 was a late marker of cell-cycle entry; Ki-67 mRNA oscillated with highest levels in G₂ while protein levels increased throughout the cell cycle, peaking in mitosis. Inhibition of CDK4/CDK6 revealed proteasome-mediated Ki-67 degradation in G₁ After cell-cycle exit, low-level Ki-67 expression persisted but was undetectable in fully quiescent differentiated cells or senescent cells. CDK4/CDK6 inhibition in vitro and in tumors in mice caused G₁ cell-cycle arrest and eliminated Ki-67 mRNA in RB1-positive cells but had no effect in RB1-negative cells, which continued to proliferate and express Ki-67. Thus, Ki-67 expression varies due to cell-cycle regulation, but it remains a reliable readout for effects of CDK4/CDK6 inhibitors on cell proliferation. Cancer Res; 77(10); 2722-34. ©2017 AACR.