Disclosed are methods of determining activity of CDK4 and CDK6 variants upon exposure to CDK inhibitors, methods for determining activity of a Rb variant, methods for determining the activity of a p16 variant in a cell, and methods for determining the sensitivity of a CDK4 variant or a CDK6 variant to p16 in a cell. Stable cell lines for determining activity of CDK4 variants, CDK6 variants, Rb variants, and p16 variants are also disclosed.