In filamentous fungi, het loci (for heterokaryon incompatibility) are believed to regulate self/nonself-recognition during vegetative growth. As filamentous fungi grow, hyphal fusion occurs within an individual colony to form a network. Hyphal fusion can occur also between different individuals to form a heterokaryon, in which genetically distinct nuclei occupy a common cytoplasm. However, heterokaryotic cells are viable only if the individuals involved have identical alleles at all het loci [1].

1: Evidence for balancing selection operating at the het-c heterokaryon incompatibility locus in a group of filamentous fungi. Wu J, Saupe SJ, Glass NL; Proc Natl Acad Sci U S A 1998;95:12398-12403. PMID:9770498