E3 ubiquitin-protein ligase. Acts as a negative coregulator for LIM homeodomain transcription factors by mediating the ubiquitination and subsequent degradation of LIM cofactors LDB1 and LDB2 and by mediating the recruitment the SIN3a/histone deacetylase corepressor complex. Ubiquitination and degradation of LIM cofactors LDB1 and LDB2 allows DNA-bound LIM homeodomain transcription factors to interact with other protein partners such as RLIM. Plays a role in telomere length-mediated growth suppression by mediating the ubiquitination and degradation of TERF1. By targeting ZFP42 for degradation, acts as an activator of random inactivation of X chromosome in the embryo, a stochastic process in which one X chromosome is inactivated to minimize sex-related dosage differences of X-encoded genes in somatic cells of female placental mammals. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Interacts with LIM/homeobox factors such as LHX3. Interacts with LDB1, LDB2 and SIN3A (By similarity). Interacts with LIMK1 (By similarity). Interacts (via N-terminus) with TERF1. Interacts (via C-terminus) with ESR1. Expressed in many tissues. The disease is caused by variants affecting the gene represented in this entry. Acts as a positive coregulator of ESR1-mediated transcription in breast cancer cells. Belongs to the RNF12 family.