Fig. (5). Examples arrest at each stage of the cell cycle due to inhibition of specific kinases Inhibition of the cell cycle using kinase inhibitor fell into four main categories demonstrated by the above four compounds. CDK4 inhibition led to a G1 arrest phenotype, CDK2 inhibition led to an S phase arrest phenotype, CDK1 inhibition led to a G2 arrest phenotype, and inhibition of PLK1 demonstrates one form of mitotic arrest phenotype.