Cyclins are important in regulating the cell cycle through their formation of enzymatic complexes with various cyclindependent kinases. The activity of the cyclin D-dependent kinases is controlled by inhibitors such as the INK4 family, which includes INK4a, INK4b, INK4c, and INK4d. The p19 protein, also known as INK4d was first identified in a yeast two-hybrid system screened for CDK4 binding proteins. Human p19 shares 86% identity at the amino acid level with murine p19, and is ~44% identical to each of the other human INK4 family members.