Human splicing factor SF3a is a part of the 17 S U2 snRNP (small nuclear ribonucleoprotein), which interacts with the pre-mRNA branch site early during spliceosome formation. The SF3a subunits of 60, 66 and 120 kDa are all required for SF3a function in vitro. Depletion of individual subunits from HeLa cells by RNA interference results in a global inhibition of splicing, indicating that SF3a is a constitutive splicing factor. Structure-function analyses have defined domains necessary for interactions within the SF3a heterotrimer, association with the U2 snRNP and spliceosome assembly. Studies aimed at the identification of regions in SF3a60 and SF3a66, required for proper intracellular localization, have led to a model for the final steps in U2 snRNP biogenesis and the proposal that SF3a is incorporated into the U2 snRNP in Cajal bodies.