Involved in pre-mRNA splicing. May function at or prior to the first catalytic step of splicing at the catalytic center of the spliceosome. May do so by stabilizing the catalytic center or the position of the RNA substrate (By similarity). Belongs to the 40S cdc5-associated complex (or cwf complex), a spliceosome sub-complex reminiscent of a late-stage spliceosome composed of the U2, U5 and U6 snRNAs and at least brr2, cdc5, cwf2/prp3, cwf3/syf1, cwf4/syf3, cwf5/ecm2, spp42/cwf6, cwf7/spf27, cwf8, cwf9, cwf10, cwf11, cwf12, prp45/cwf13, cwf14, cwf15, cwf16, cwf17, cwf18, cwf19, cwf20, cwf21, cwf22, cwf23, cwf24, cwf25, cwf26, cyp7/cwf27, cwf28, cwf29/ist3, lea1, msl1, prp5/cwf1, prp10, prp12/sap130, prp17, prp22, sap61, sap62, sap114, sap145, slu7, smb1, smd1, smd3, smf1, smg1 and syf2. Belongs to the CWC21 family.