Introduction. Due to the aberrant deregulation of the CDK4/6-INK4-RB pathway in breast cancer, CDK4/6 inhibitors are now a cornerstone of first line treatment of metastatic breast cancer showing an increase of progression free survival and overall survival. However, resistance to treatment eventually occur. We aimed at deciphering the molecular mechanisms of resistance to CDK4-6 inhibitors and at evaluating associations of therapies that could restore the sensitivity to the treatment. Methods. To identify potential DNA methylation signatures associated with the acquired resistance to CDK4/6 inhibitors, we established a MCF7 model, resistant to CDK4/6 inhibitors that we also used in Swiss nude mice via xenograft experiments. qMSRE were used to analyze the methylation status of genes encoding for BCL2 family members in cells, but also on circulating tumor DNA (ctDNA) in our in vivo model. Results. Using acquired resistant MCF7 cells to CDK4/6 inhibitors, our work demonstrate that these cells harbor a phenotype of high apopto-sensitivity induced by a BH3-mimetic drug. We next discern that DNMT3A and TET2 are the molecular determinants of this phenomenon via the epigenetic modulation of DNA methylation profile of genes encoding for BCL2 family. Using in vivo model, we noted that the ctDNA dynamic of the DNA methylation level of genes encoding for BCL2 family members is associated with the emergence of the resistance to CDK4/6 inhibitor demonstrated by the absence of tumor growth inhibition. Our experiments also indicated that the administration of a BH3-minetic drug at this time point restores the inhibition of tumor growth. Conclusion. This work provides the evidence that epigenetic modulation (methylation level) of key genes detected in ctDNA is associated with resistance to CDK4-6 inhibitors. The BH3-mimetic drugs are seem really appealing in that context.