Required for developmental progression after cells of opposite mating types fuse with one another, essential for processes common to both dikaryotic filament formation and monokaryotic fruiting. A direct target for transcription factors Sxi1-alpha and Sxi2-a. Expressed abundantly under sexual development conditions. Expression is not seen in haploid cells where both copies of the gene are deleted and there is no discernible phenotype with respect to haplotype growth in strains with complete deletions. However, there is a complete absence of sexual development and strains fail to form dikaryotic filaments/spores. Where only a single copy is present in a wild-type and mutant mating pair sexual development is indistinguishable from a wild-type mating pair. Expression is up-regulated during sexual development.