Associated component of the WMM complex, a complex that mediates N6-methyladenosine (m6A) methylation of RNAs, a modification that plays a role in the efficiency of mRNA splicing and RNA processing (PubMed:29507755). Required for accumulation of METTL3 and METTL14 to nuclear speckle (PubMed:24316715, PubMed:24407421, PubMed:24981863). Acts as a mRNA splicing regulator (PubMed:12444081). Regulates G2/M cell-cycle transition by binding to the 3' UTR of CCNA2, which enhances its stability (PubMed:17088532). Impairs WT1 DNA-binding ability and inhibits expression of WT1 target genes (PubMed:17095724). Component of the WMM complex, a N6-methyltransferase complex composed of a catalytic subcomplex, named MAC, and of an associated subcomplex, named MACOM (PubMed:24407421, PubMed:24981863, PubMed:24316715, PubMed:29507755, PubMed:29506078, PubMed:29348140). The MAC subcomplex is composed of METTL3 and METTL14 (PubMed:29507755). The MACOM subcomplex is composed of WTAP, ZC3H13, CBLL1/HAKAI, VIRMA, and, in some cases of RBM15 (RBM15 or RBM15B) (PubMed:29507755). Interacts with WT1 (PubMed:11001926, PubMed:17095724). Also a component of a MACOM-like complex, named WTAP complex, composed of WTAP, ZC3H13, CBLL1, VIRMA, RBM15, BCLAF1 and THRAP3 (PubMed:24100041). Mainly nuclear with some fraction located in the cytoplasm. ZC3H13 is required to anchor component of the MACOM subcomplex, such as VIRMA, in the nucleus. Ubiquitously expressed. In smooth muscle cells, up-regulated after serum withdrawal, when cells become mature and non proliferative. Belongs to the fl(2)d family.