Inhibitors of mammalian G1 cyclin-dependent kinases (cdks) play a crucial role in regulating the G1 phase of the cell cycle. The discovery of mammalian G1 cyclins and their associated cdks provided insights into how cells progress through the G1 phase. Recent studies have focused on understanding the negative regulators, specifically cdk inhibitors, which control the activity of these kinases. G1 cyclin-dependent kinases (cdks) are regulated by cyclins, which form functional holoenzymes that phosphorylate key substrates to facilitate cell cycle transitions. D-type cyclins, such as D1, D2, and D3, are synthesized during the G1 phase and are rate-limiting for S-phase entry. They assemble with cdk4 and cdk6, and their activity is regulated by mitogens. Once assembled, cyclin D-bound cdk5 must be phosphorylated by a cdk-activating kinase (CAK) to become catalytically active. D-type cyclins are short-lived, and their withdrawal during G1 prevents their steady accumulation, correlating with the failure of growth factor-deprived cells to progress past the restriction (R) point. Cyclin E, expressed later in G1, forms complexes with cdk2 and is also regulated by CAK. Cyclin E-dependent kinases phosphorylate the retinoblastoma protein (pRb), releasing transcription factors like E2F, which activate genes necessary for S-phase entry. pRb phosphorylation at the R point cancels its growth-suppressive function, allowing these transcription factors to activate genes required for S-phase entry. p21 (Cip1, Waf1, Sdi1, Cap20) is a key cdk inhibitor that binds to cdk2, cdk4, and cdk1, inhibiting their activity and inducing cell cycle arrest. p21 is induced in response to mitogenic signals and is involved in DNA damage response, terminal differentiation, and cell senescence. It can also inhibit DNA replication by interacting with PCNA, a subunit of DNA polymerase-δ. p21's cdk-inhibitory and PCNA-binding domains are functionally distinct and reside in the amino-terminal and carboxy-terminal halves of the molecule, respectively. p27 (Kip1) and p57 (Kip2) are other cdk inhibitors that bind to cyclin-cdk complexes, inhibiting their activity. p27 is expressed in quiescent cells and is involved in cell cycle arrest. p57 is more tissue-restricted than p27 and is involved in cell cycle regulation in various tissues. p16 (INK4a) is a cdk inhibitor that binds to cdk4 and cdk6, preventing their activity. It is involved in cell cycle control and tumor suppression. The Ink4 familyInhibitors of mammalian G1 cyclin-dependent kinases (cdks) play a crucial role in regulating the G1 phase of the cell cycle. The discovery of mammalian G1 cyclins and their associated cdks provided insights into how cells progress through the G1 phase. Recent studies have focused on understanding the negative regulators, specifically cdk inhibitors, which control the activity of these kinases. G1 cyclin-dependent kinases (cdks) are regulated by cyclins, which form functional holoenzymes that phosphorylate key substrates to facilitate cell cycle transitions. D-type cyclins, such as D1, D2, and D3, are synthesized during the G1 phase and are rate-limiting for S-phase entry. They assemble with cdk4 and cdk6, and their activity is regulated by mitogens. Once assembled, cyclin D-bound cdk5 must be phosphorylated by a cdk-activating kinase (CAK) to become catalytically active. D-type cyclins are short-lived, and their withdrawal during G1 prevents their steady accumulation, correlating with the failure of growth factor-deprived cells to progress past the restriction (R) point. Cyclin E, expressed later in G1, forms complexes with cdk2 and is also regulated by CAK. Cyclin E-dependent kinases phosphorylate the retinoblastoma protein (pRb), releasing transcription factors like E2F, which activate genes necessary for S-phase entry. pRb phosphorylation at the R point cancels its growth-suppressive function, allowing these transcription factors to activate genes required for S-phase entry. p21 (Cip1, Waf1, Sdi1, Cap20) is a key cdk inhibitor that binds to cdk2, cdk4, and cdk1, inhibiting their activity and inducing cell cycle arrest. p21 is induced in response to mitogenic signals and is involved in DNA damage response, terminal differentiation, and cell senescence. It can also inhibit DNA replication by interacting with PCNA, a subunit of DNA polymerase-δ. p21's cdk-inhibitory and PCNA-binding domains are functionally distinct and reside in the amino-terminal and carboxy-terminal halves of the molecule, respectively. p27 (Kip1) and p57 (Kip2) are other cdk inhibitors that bind to cyclin-cdk complexes, inhibiting their activity. p27 is expressed in quiescent cells and is involved in cell cycle arrest. p57 is more tissue-restricted than p27 and is involved in cell cycle regulation in various tissues. p16 (INK4a) is a cdk inhibitor that binds to cdk4 and cdk6, preventing their activity. It is involved in cell cycle control and tumor suppression. The Ink4 family