Involved in pre-mRNA splicing as component of the activated spliceosome. Part of the activated spliceosome B/catalytic step 1 spliceosome, one of the forms of the spliceosome which has a well-formed active site but still cannot catalyze the branching reaction and is composed of at least 52 proteins, the U2, U5 and U6 snRNAs and the pre-mRNA. Belongs to the CWC26 family.