Worldwide, breast cancer (BC) is the second most common cancer. Pharmacologically targeting cyclin-dependent kinase 4 and 6 (CDK4 & 6) has proven to be a successful therapeutic approach in patients with estrogen receptor-positive (ER+) breast cancer. Abemaciclib is the first FDA-approved CDK4 & 6 inhibitor (CDK4 & 6i) approved for the adjuvant treatment of HR+, HER2–, node-positive early breast cancer (EBC) at high risk of recurrence and a Ki-67 score ≥20%. Differences have been observed in both efficacy and severity of neutropenia among the available CDK4 & 6i, generating interest in a possible mechanistic explanation. In their study published in...