Involved in pre-mRNA splicing. May function at or prior to the first catalytic step of splicing at the catalytic center of the spliceosome, together with ISY1. May do so by stabilizing the catalytic center or the position of the RNA substrate. Associates with the NTC complex (or PRP19-associated complex), composed of at least CEF1, CLF1, ISY1, NTC20, SNT309, SYF1, SYF2, and PRP19. The NTC complex associates with the spliceosome after the release of the U1 and U4 snRNAs and forms the CWC spliceosome subcomplex (or CEF1-associated complex) reminiscent of a late-stage spliceosome composed also of the U2, U5 and U6 snRNAs and at least BUD13, BUD31, BRR2, CDC40, CUS1, CWC2, CWC15, CWC21, CWC22, CWC23, CWC24, CWC25, CWC27, ECM2, HSH155, IST3, LEA1, MSL1, PRP8, PRP9, PRP11, PRP21, PRP22, PRP45, PRP46, SLU7, SMB1, SMD1, SMD2, SMD3, SMX2, SMX3, SNU114, SPP2, RSE1 and YJU2. Interacts directly with PRP8 (via SCwid domain). Interacts directly with SNU114 (via C-terminus). Present with 556 molecules/cell in log phase SD medium. Belongs to the CWC21 family.