A gene on chromosome 2q33.1 that encodes subunit 1 of the splicing factor SF3B, which is required for "A" complex assembly by the stable binding of U2 snRNP to the branchpoint sequence in pre-mRNA. Binding of the SF3A/SF3B complex upstream of the branch site may anchor U2 snRNP to pre-mRNA. SF3B1 may be involved in assembling the "E" complex.