RNA-binding protein that acts as a key regulator of N6-methyladenosine (m6A) methylation of RNAs, thereby regulating different processes, such as hematopoietic cell homeostasis, alternative splicing of mRNAs and X chromosome inactivation mediated by Xist RNA (PubMed:29535189). 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:29535189). Plays a key role in m6A methylation, possibly by binding target RNAs and recruiting the WMM complex (PubMed:29535189). 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 (By similarity). Required for the development of multiple tissues, such as the maintenance of the homeostasis of long-term hematopoietic stem cells and for megakaryocyte (MK) and B-cell differentiation (PubMed:17283045, PubMed:17376872, PubMed:18981216, PubMed:25468569). Regulates megakaryocyte differentiation by regulating alternative splicing of genes important for megakaryocyte differentiation; probably regulates alternative splicing via m6A regulation (By similarity). Required for placental vascular branching morphogenesis and embryonic development of the heart and spleen (PubMed:18981216). Acts as a regulator of thrombopoietin response in hematopoietic stem cells by regulating alternative splicing of MPL (PubMed:25468569). May also function as an mRNA export factor, stimulating export and expression of RTE-containing mRNAs which are present in many retrotransposons that require to be exported prior to splicing (By similarity). High affinity binding of pre-mRNA to RBM15 may allow targeting of the mRNP to the export helicase DBP5 in a manner that is independent of splicing-mediated NXF1 deposition, resulting in export prior to splicing (By similarity). May be implicated in HOX gene regulation (By similarity). Component of the WMM complex, a N6-methyltransferase complex composed of a catalytic subcomplex, named MAC, and of an associated subcomplex, named MACOM (PubMed:29535189). The MAC subcomplex is composed of METTL3 and METTL14 (PubMed:29535189). The MACOM subcomplex is composed of WTAP, ZC3H13, CBLL1/HAKAI, VIRMA, and, in some cases of RBM15 (RBM15 or RBM15B) (PubMed:29535189). Also a component of a MACOM-like complex, named WTAP complex, composed of WTAP, ZC3H13, CBLL1, VIRMA, RBM15, BCLAF1 and THRAP3 (By similarity). Interacts with RBPJ (PubMed:17283045). Interacts (via SPOC domain) with SETD1B (By similarity). Interacts with NXF1, the interaction is required to promote mRNA export (By similarity). Interacts with SF3B1 (By similarity). Colocalizes at the nuclear pore with DBP5 and NXF1. Methylated at Arg-577 by PRMT1, leading to promote ubiquitination by CNOT4 and subsequent degradation by the proteasome. Ubiquitinated by CNOT4 following methylation at Arg-577 by PRMT1. Embryonic lethality around E9.5 (PubMed:17376872, PubMed:18981216). Early embryos show growth retardation and incomplete closure of the notochord, as well as placental defects in the spongiotrophoblast and syncytiotrophoblast layers, resulting in an arrest of vascular branching morphogenesis (PubMed:18981216). Conditional knockout mice lacking Rbm15 within the hematopoietic compartment display a loss of peripheral B-cells due to a block in pro/pre-B differentiation, as well as a myeloid and megakaryocytic expansion in spleen and bone marrow (PubMed:17376872). Belongs to the RRM Spen family. Contaminating sequence. Extended N-terminus.