The family of SR factors all contain one or more RNA recognition motifs (RRM) and an SR-rich domain. They are not only essential for constitutive splicing, but also regulate splicing in a concentration-dependent manner by influencing the selection of alternative splice sites. Splicing factor arginine/serine-rich 15 (SFRS15), also designated CTD-binding SR-like protein RA4, contains one RRM and one SR-rich domains. SFRS15 interacts with C-terminal repetitive domain (CTD) of Pol II and is believed to functionally and physically link transcription and pre-mRNA processing. Localized to the nucleus, SFRS15 is expressed as two isoforms produced by alternative splicing.