The document discusses two families of cyclin-dependent kinase inhibitors (CDKIs) - the CIP family and the INK family. The CIP family includes proteins like p21cip, p27kip2, and p57kip2 that inhibit cyclin A-CDK2 activity and must be degraded for DNA replication to begin. The INK family includes proteins like p16 that specifically interact with and inhibit CDK4/CDK6, blocking their interaction with cyclin D, phosphorylation of Rb protein, and entry into the S phase of the cell cycle. Mutation of the INK4 protein p16 can lead to tumor development.