many of these species likely have an as yet unidentified sexual stage. Sexual reproduction occurs in two fundamentally different ways in fungi. These are outcrossing (in heterothallic fungi) in which two different individuals contribute nuclei, and self-fertilization or selfing (in homothallic fungi) in which both nuclei are derived from the same individual. In recent years, sexual cycles have been discovered in numerous species previously thought to be asexual. These discoveries reflect recent experimental focus on species of particular relevance to humans. A. fumigatus is the most common species to cause disease in immunodeficient humans. In 2009, A. fumigatus was shown to have a heterothallic, fully functional sexual cycle. Isolates of complementary mating types are required for sex to occur. A. flavus is the major producer of carcinogenic aflatoxins in crops worldwide. It is also an opportunistic human and animal pathogen, causing aspergillosis in immunocompromised individuals. In 2009, a sexual state of this heterothallic fungus was found to arise when strains of opposite mating types were cultured together under appropriate conditions. A. lentulus is an opportunistic human pathogen that causes invasive aspergillosis with high mortality rates. In 2013, A. lentulus was found to have a heterothallic functional sexual breeding system. A. terreus is commonly used in industry to produce important organic acids and enzymes, and was the initial source for the cholesterol-lowering drug lovastatin. In 2013, A. terreus was found to be capable of sexual reproduction when strains of opposite mating types were crossed under appropriate culture conditions. These findings with Aspergillus species are consistent with accumulating evidence, from studies of other eukaryotic species, that sex was likely present in the common ancestor of all eukaryotes. A. nidulans, a homothallic fungus, is capable of self-fertilization. Selfing involves activation of the same mating pathways characteristic of sex in outcrossing species, i.e. self-fertilization does not bypass required pathways for outcrossing sex, but instead requires activation of these pathways within a single individual. Among those Aspergillus species that exhibit a sexual cycle, the overwhelming majority in nature are homothallic (self-fertilizing). This observation suggests Aspergillus species can generally maintain sex though little genetic variability is produced by homothallic self-fertilization. A. fumigatus, a heterothallic (outcrossing) fungus that occurs in areas with widely different climates and environments, also displays little genetic variability either within geographic regions or on a global scale, again suggesting sex, in this case outcrossing sex, can be maintained even when little genetic variability