Involved in pre-mRNA splicing. May be a component of the spliceosome. Detected in intestine, pharynx, vulval muscles and body wall muscles. Developmental arrest at larval stage L1 or L2. RNAi-mediated knockdown results in aberrant splicing of tos-1 mRNA with increased intron 1 retention and exon 3 skipping, leading to increased expression levels of tos-1 isoforms 1 and 2. However there is no obvious recognition of the cryptic 3' splice site in tos-1 intron 1. Belongs to the MFAP1 family.