As CDK4/6 inhibitors including ribociclib, abemaciclib, and palbociclib start to occupy a key role in the treatment of HR+, HER2− breast cancer [1], our interest focused on creating a ribociclib-resistant cell line from the widely used and characterized ER+, HER2− CAMA-1 cell line [45, 46]. The gene discussed is ESR1; the disease is breast cancer.