The button mushroom (Agaricus bisporus)is one of the world’s most cultivated mushroom species. In spite of its economic importance, new cultivars by outbreeding have been hardly produced in the last 40 years. One of the reasons for this lack of breeding effort is the difficulty to introduce new traits. This is caused by the atypical meiosis in button mushrooms. Recombination between homologous chromosomes is restricted to chromosome ends resulting in the inheritance of nearly parental type of chromosomes. In addition, instead of four spores, most basidia produce two spores in which non-sister nuclei are paired. As a result, these spores are fertile and each produce mushrooms that are genetically and phenotypically very similar to the parental line. Genetic analysis has shown that most present-day white commercial cultivars are derived in this way from the first hybrids released in 1980. These were generated by outcrossing and required a considerable investment. The selection of fertile single spore cultures from protected varieties and the use of these to generate new cultivars should be considered as the generation of essentially derived varieties (EDV) similar to how derived varieties in plant cultivars are defined. A working group of 4 spawn/breeding companies and two research groups has been formed to come to a consensus what should be considered as essentially derived varieties in cultivars of edible fungi. The initiative of this group is supported by the international organization of plant protection (UPOV), the European agency that manage the system of plant variety rights (CPVO) and the ISMS. Next generation sequencing technologies can support the identification of copied, derived and genially new cultivars.