lin-15 is a complex locus containing two non-overlapping transcripts, lin-15A and lin-15B, separated by less than 150 bp and transcribed in the same direction. LIN-15A and B proteins are novel and have a THAP zinc-finger-like (C2CH) motif. The lin-15A transcript is processed with the addition of an SL1 or and SL2 trans-splice leader. lin-15B has only been shown to have an SL1 added. These features suggest the lin-15 locus encodes a single polycistronic mRNA. Alleles of lin-15 were first obtained based on the Multivulva (Muv) phenotype. More alleles were identified in screens for vulval cell lineage mutants, where all six vulval precursor cells (VPCs) adopt vulval fates, causing the Muv phenotype. Initial studies of these alleles showed lin-15 to have two independently mutable activities, A and B; A and B class Muv genes were defined by synthetic genetic interactions with lin-8 and lin-9, respectively. Homozygous class A or B mutants do not exhibit any vulval phenotypes whereas A+B double homozygotes are Syn-Muv. While some lin-15 alleles fall into either the A OR B classes, for example, n767 and n744 respectively, most of the alleles are Muv as homozygotes alone and hence are AB mutants, for example, n765. Molecular analysis showed that the lin-15 locus comprised two independent transcripts with the downstream transcript encoding lin-15A function and the upstream transcript encoding lin-15B function. A and B mutant alleles alter the downstream and upstream functions, respectively, and AB alleles affect both transcripts. Further, all single homozyogus Muv alleles proved to be rearrangements or deletions in the lin-15 locus. Although A and B functions are genetically redundant, A function can not rescue B function and thus A and B are not molecularly redundant. Fragments of the lin-15 locus that rescue either lin-15A or lin-15B are common and useful coinjection transgenic markers when using a lin-15(n765tsAB) mutant background.