We report here the results of a retrospective analysis of adherence to abemaciclib plus endocrine therapy in a cohort of women with hormone receptor-positive (HR+), epidermal growth factor 2 negative (HER2−) breast cancer. Methods: Abemaciclib adherence was computed as the ratio between the total number of cycles/months that medication was supplied and the months between the first and the last prescription. This evidence concerns the gene ERBB2 and breast carcinoma.