Heterogeneous nuclear ribonucleoproteins (hnRNPs) constitute a set of polypeptides that contribute to pre-mRNA processing and transport. hnRNPs also bind heterogeneous nuclear RNA (hnRNA), the transcripts produced by RNA polymerase II. SAFB (scaffold attachment factor B) is a nuclear matrix-associated protein that binds to matrix- or scaffold-associating regions (MARs or SARs) on DNA and interacts with RNA polymerase II and serine-/arginine-rich RNA processing factors (SR proteins). SAFB, also designated HAP (hnRNP A1 associated protein) and HET (HSP 27-ERE-TATA-binding protein) is a proven hnRNP protein that has a speckled distribution in the nucleus and, in response to stress agents such as heat shock, is recruited to a few, large nuclear granules, called perichromatin granules. SAFB also binds to the estrogen receptor (ER) and is expressed in several breast cancer cell lines at varying levels. Subsequently, SAFB may play a role in breast cancer by mediating cellular proliferation and division. This antibody specifically reacts whith the isoform 1 of human SAFB protein. SF3A1, also known as SF3a120, is a 793 amino acid protein, which is a Subunit of the splicing factor SF3A required for 'A' complex assembly formed by the stable binding of U2 snRNP to the branchpoint sequence (BPS) in pre-mRNA. SF3A1 also be involved in the assembly of the 'E' complex. The calculated molecular weight of SAFB is 103 kDa, but the modified SAFB is about 130-175 kDa.(PMID: 28469965, PMID: 26273616 )