MS28 is selective for cyclin D1/D3 and CDK4/6. (A) Left, MS28, but not MS28N1, degrades cyclin D1/3 and CDK4/6, and reduces the cyclin A2 level, but not cyclin D2/B1 and CDK2 in Calu-1 cells (4 h-treatment at 3 μM). WB data shown are representative of two biological repeats. Right, quantification of the relative abundance of cyclins and CDKs analyzed in the WB upon DMSO, MS28, or MS28N1 treatment. P-values were calculated relative to the DMSO control for each protein. ****P < 0.0001, ***P < 0.001, and **P < 0.01. (B) MS28 is selective for CDK6 over a panel of 57 other kinases at 1 μM concentration. Data are the means ± SD from duplicate experiments.