Splicing, the removal of introns from pre-mRNA, is mediated by spliceosomal complexes and occurs in two distinct catalytic steps. The first step involves cleavage of the 5' exon and the production of a lariat intermediate. In the second step, the 3' splice site is cleaved and the exons are fused with concomitant release of the intron lariat. The spliceosome contains multiple snRNPs and a number of non-snRNP splicing factors. Four yeast proteins (Prp16p, Prp17p, Prp18p, and Slu7p) function exclusively in the second catalytic step. Human homologs have been identified for Prp16p (hPrp16), Prp17p (hPrp17), Prp18p (hPrp18) and Slu7 (hSlu7). hSlu7 contains a zinc knuckle motif similar to the yeast Slu7. This domain is present in retroviral nucleocapsid proteins and in several splicing factors. hSlu7 associates with the spliceosome late in the splicing pathway during recognition of the 3' splice site. During step II of 3' splicing, hSlu7 tightly binds to exon 1 in the spliceosome and helps specify attack on the correct adenine-guanine dinucleotide, located 18 to 40 nucleotides downstream of the branch site.