Cyclin dependent kinases, or CDKs, are an intensively investigated family of evolutionarily conserved protein kinases that orchestrate the cell cycle and gene transcription. One of the first irreversible CDK inhibitors reported is THZ1, which inhibits CDK7 by docking in the active site and covalently modifying the nearby Cys312 residue. Subsequent studies with THZ1 revealed the therapeutic potential of targeting CDK7 in many aggressive cancers, including MYCN-amplified neuroblastoma 182 , small-cell lung cancer 183 , and triple-negative breast cancer. At higher concentration, THZ1 also demonstrates some activity against closely related kinases CDK12 and CDK13. Based on the THZ1 scaffold, newer covalent inhibitors, SY-1365 and THZ531, with better selectivity profiles have been identified. SY-1365 is currently being investigated for the treatment of ovarian and breast cancers. THZ531 is a covalent inhibitor selective for CDK12 and CDK13, sparing CDK7 activity. Treatment of Jurkat T-ALL cells with THZ531 diminished pSer2 and decreased the expression of DDR and super enhancer-associated transcription factor genes. Neuroblastoma and lung cancer cells develop resistance to THZ1 and THZ531 through upregulation of the ABCB1 or ABCG2 drug transporters, implying that these compounds are substrates for these proteins. Efforts to overcome drug resistance to THZ1 and THZ531 led to the discovery of another generation of irreversible CDK inhibitors, compound E9. A highly selective CDK7 covalent inhibitor, YKL-5-124, triggered cell-cycle arrest without global effects on transcription. 192 Also, in contrast to THZ1, YKL-5-124 treatment only modestly affected bulk Pol II (RNA polymerase II) phosphorylation on CTD residues Ser2, Ser5 and Ser7. YKL-5-124 resulted from fusing the structure of THZ1 with PF 3758309, a PAK4 inhibitor and optimizing the resultant compound to maximize interactions at CDK7. Interestingly, the biochemical and cellular effects of YKL-5-124 more closely resemble those produced by allele-specific inhibition of CDK7 as in human colon cancerderived cells. Another combinatorial strategy for triggering cancer cell deathinducing transcriptional dependency with agents that activate p53 while selectively inhibiting CDK7-recently emerged in studies with CDK7 as/as colon-cancer-derived cells and was recapitulated in wild-type tumor cells with another CDK7-selective covalent inhibitor, YKL-1-116. Genetic depletion of cyclin-dependent kinase 12 (CDK12) or selective inhibition of an analog-sensitive CDK12 reduces the expression of DNA damage repair genes. MFH290, a highly selective covalent inhibitor of CDK12/13, was generated by combining structural features of CDK12/13 covalent inhibitor THZ531 with the previously reported noncovalent pan-CDK inhibitor SNS032. 196 MFH290 forms a covalent bond with Cys1039 of CDK12, exhibiting excellent CDK12 selectivity and inhibits the phosphorylation of Ser2 in the Cterminal domain of Pol II thereby reducing the expression of key DNA damage repair genes. 197 MFH290 demonstrated a sustained CDK12 inhibition-induced phenotype including strong antiproliferative effect on cancer cells, a transcriptional defect for DDR genes, and a combinatorial effect with PARP inhibition. 197 BSJ-01-175, another CDK12/13 covalent inhibitor, resulted from structure-activity relationships (SAR) efforts on THZ531 and is reported to possess a higher degree of selectivity for CDK12/13 as compared to its parent THZ531 even at as high concentrations as 5 µM. Additionally, and most importantly BSJ-01-175 shows potent in vivo efficacy. FMF-04-159-2, a CDK14-specific covalent inhibitor, and its reversible analog were used to characterize the cellular consequences of covalent CDK14 inhibition, including an unbiased investigation using phospho-proteomics. This investigation suggested that CDK14 plays a supporting role in cell-cycle regulation, particularly mitotic progression, and identified putative CDK14 substrates. Together, these results represent an important step forward in understanding the cellular consequences of inhibiting CDK14 kinase activity. 8. Euchromatic histone lysine methyltransferase 2/lysine methyltransferases (G9alike protein) (G9a/GLP) G9a (also known as euchromatic histone lysine methyltransferase 2; EHMT2) and its closely related paralogue GLP (G9a-like protein, also known as euchromatic histone lysine methyltransferase 1; EHMT1) can catalyze the methylation of both histone and nonhistone substrates. G9a overexpression is associated with proliferation and metastasis in several types of cancer including brain, breast, ovarian, lung, bladder, melanoma, and colorectal cancer. Moreover, it has been shown that G9a is involved in embryonic stem cell maintenance and T-cell differentiation and is implicated in other diseases such as Alzheimer's disease (AD) , sickle cell disease 211 and Prader-Willi syndrome. 212 MS8511 213 the first G9a/GLP covalent inhibitor reported, reduced histone H3 lysine 9 (H3K9me2) methylation levels and showed enhanced antiproliferative activity over related noncovalent inhibitors. The acrylamide warhead of MS8511 is thought to interact with Cys1098 and Cys1186 in G9a and GLP, respectively. There appears to be a slight preference for G9a over GLP, due to faster modification/kinetics, by MS8511. Overall, MS8511 is a highly potent, selective, and cell-active covalent inhibitor of G9a and GLP which could serve as a useful chemical tool for investigating the physiological and pathophysiological functions of G9a and GLP. 213