RNA-binding protein that acts as a key regulator of N6- methyladenosine (m6A) methylation of RNAs, thereby regulating different processes, such as alternative splicing of mRNAs and X chromosome inactivation mediated by Xist RNA (PubMed:16129689, PubMed:27602518). 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:27602518). Plays a key role in m6A methylation, possibly by binding target RNAs and recruiting the WMM complex (PubMed:27602518). Involved in random X inactivation mediated by Xist RNA: acts by binding Xist RNA and recruiting the WMM complex, which mediates m6A methylation, leading to target YTHDC1 reader on Xist RNA and promoting transcription repression activity of Xist (PubMed:27602518). Functions in the regulation of alternative or illicit splicing, possibly by regulating m6A methylation (PubMed:16129689). Inhibits pre-mRNA splicing (PubMed:21044963). Also functions as a mRNA export factor by acting as a cofactor for the nuclear export receptor NXF1 (PubMed:19586903). {ECO:0000269|PubMed:19586903, ECO:0000269|PubMed:21044963, ECO:0000269|PubMed:27602518, ECO:0000305|PubMed:16129689}. This is the function of RBM15B (RNA binding motif protein 15B, ENSG00000259956).