BACKGROUND: Palbociclib is a cell-cycle targeted small molecule agent used as one of the standards of care in combination with endocrine therapy for patients with hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced breast cancer. Although several gene alterations such as loss of Rb gene and amplification of p16 gene are known to be conventional resistance mechanisms to cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors, the comprehensive landscape of resistance is not yet fully elucidated. The purpose of this study is to identify the novel resistant genes to the CDK4/6 inhibitors in HR-positive HER2-negative breast cancer. METHODS: The whole genome knockout screen using CRISPR/Cas9 genome editing was conducted in MCF7 to identify resistant genes to palbociclib. The candidate genes for resistance were selected by NGS analysis and GSEA analysis and validated by cell viability assay and mouse xenograft models. RESULTS: We identified eight genes including RET, TIRAP, GNRH1, SEMA3F, SEMA5A, GATA4, NOD1, SSTR1 as candidate genes from the whole genome knockout screen. Among those, knockdown of SEMA3F by siRNA significantly and consistently increased the cell viability in the presence of CDK4/6 inhibitors in vitro and in vivo. Furthermore, the level of p-Rb was maintained in the palbociclib treated SEMA3F-downregulated cells, indicating that the resistance is driven by increased activity of cyclin kinases. CONCLUSION: Our observation provided the first evidence of SEMA3F as a regulator of sensitivity to CDK4/6 inhibitors in breast cancer. The detailed mechanisms of resistance deserve further functional studies to develop the better strategy to overcome resistance in CDK4/6 inhibitors.