Major functional domains and key sites for PTM in TRIM28. The factor contains multiple functional domains. TRIM28 binds to KRAB transcription factors through the RBCC domain in the N-terminal portion and to heterochomatin 1 (HP1) proteins through a centrally located binding motif. Gene repression is mediated through C-terminal domains including adjacent PHD and BR domains. Key posttranslational modifications (PTM) include tyrosine and serine phosphorylation sites clustered around the HP1 binding motif and sumoylation sites in the BR domain. A critically important regulatory phosphorylation site is serine 824 located in the extreme C-terminal region.