EGLN1 and cancer: BRD4 is a proline hydroxylated substrate in cancer cells, PHD2 is a key regulatory enzyme for the proline hydroxylation of BRD4, and the hydroxylation site (P536) on BRD4 is located at the junction between the phosphorylation-rich NPS domain and the lysine-rich BID domain (88). BRD4 proline hydroxylation not only regulates the interaction between BRD4 and specific binding proteins, such as CDK9, CCNT1, but also affects BRD4-mediated gene transcription by recruiting P-TEFb to the promoter and activating RNA polymerase II (RNAPII)-mediated transcriptional activity (88).