Essential protein involved in pre-mRNA cis- and trans-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. Associates with the NTC complex (or PRP19-associated complex) (By similarity). The NTC complex associates with the spliceosome after the release of the U1 and U4 snRNAs and forms the CWC spliceosome subcomplex reminiscent of a late-stage spliceosome (By similarity). Associates specifically with U5-containing snRNPs. Found predominantly in the nucleus. Belongs to the CWC21 family.