Fungi do not have different sexes characterized by specific morphological organs but control sexual development by physiological differences defined by mating types. While other lineages in the fungal kingdom have only two distinct mating types, multiple mating types are common in the Basidiomycetes, presumably for outcomes related to enhanced outbreeding. Mating types in Basidiomycetes are determined by genes that encode two types of homeodomain transcription factors (HD genes) and by genes that encode lipopeptide pheromones and pheromone receptors (PR genes). In the tetrapolar mushroom Coprinopsis cinerea, these genes reside at two distinct loci on different chromosomes and regulate distinct steps in dikaryon formation and maintenance and fruiting body formation for sexual reproduction by controlling the expression of large sets of genes. The matA (HD) locus in C. cinerea has an estimated 160 alleles in nature, and the matB (PR) locus has 79. The respective products of both loci interact with each other in a lock-and-key manner. Proteins encoded in allelic sets of mating type genes recognize each other and interact in order to either form a functional transcription factor complex that is transported into the nucleus (matA proteins) or to initiate a pheromone response signaling cascade (matB proteins). Products encoded by the same matA or matB allele do not interact, which is crucial to the functionality of the two systems controlling sexual development. Multiple alleles of mating type genes arose in evolution through the evolution of distinct DNA sequences and the resulting encoded proteins perform the tasks of recognition and discrimination. In a second modus of multiplication, paralogous sets of HD and PR genes evolved by gene duplications and subsequent sequence diversification in order to mediate functional independency and respective allele variation. In C. cinerea, there are thus three paralogous groups of functional mating type genes at both the matA and the matB locus. Because groups of paralogous genes are freely interchangeable, only a few alleles of each of the actual gene groups (between 2 and 9 are known per group) are required to generate the large number of distinct matA and matB mating types found in nature. As deduced from the ever increasing number of genome projects, the basic principles of evolution of multiple mating types as unraveled in C. cinerea by Lorna A. Casselton and her coworkers are to different degrees also operative in other mushroom species.