The function of Ensembl gene identifier ENSG00000063978 (RNF4, ring finger protein 4) is as follows. E3 ubiquitin-protein ligase which binds polysumoylated chains covalently attached to proteins and mediates 'Lys-6'-, 'Lys-11'-, 'Lys- 48'- and 'Lys-63'-linked polyubiquitination of those substrates and their subsequent targeting to the proteasome for degradation (PubMed:18408734, PubMed:19307308, PubMed:35013556). Regulates the degradation of several proteins including PML and the transcriptional activator PEA3 (PubMed:18408734, PubMed:19307308, PubMed:20943951). Involved in chromosome alignment and spindle assembly, it regulates the kinetochore CENPH-CENPI-CENPK complex by targeting polysumoylated CENPI to proteasomal degradation (PubMed:20212317). Regulates the cellular responses to hypoxia and heat shock through degradation of respectively EPAS1 and PARP1 (PubMed:19779455, PubMed:20026589). Alternatively, it may also bind DNA/nucleosomes and have a more direct role in the regulation of transcription for instance enhancing basal transcription and steroid receptor-mediated transcriptional activation (PubMed:12885770). Catalyzes ubiquitination of sumoylated PARP1 in response to PARP1 trapping to chromatin, leading to PARP1 removal from chromatin by VCP/p97 (PubMed:35013556). {ECO:0000269|PubMed:12885770, ECO:0000269|PubMed:18408734, ECO:0000269|PubMed:19307308, ECO:0000269|PubMed:19779455, ECO:0000269|PubMed:20026589, ECO:0000269|PubMed:20212317, ECO:0000269|PubMed:20943951, ECO:0000269|PubMed:35013556}.