CDK4 and neoplasm: CDK4/6 inhibitors are a class of targeted therapies that block the activity of cyclin-dependent kinases 4 and 6 [CDK4/6], which are proteins involved in regulating cell cycle progression. CDK4/6 inhibitors, such as palbociclib, ribociclib, and abemaciclib, work by halting cancer cell division, potentially slowing tumor progression.