 Most notably, the FtM subjects not only had the variant genotype more frequently, but had an allele distribution equivalent to male controls, unlike the female controls.The paper concluded that the loss of a female-specific CYP17 T -34C allele distribution pattern is associated with FtM transsexuality.In 2013, a twin study combined a survey of pairs of twins where one or both had undergone, or had plans and medical approval to undergo, gender transition, with a literature review of published reports of transgender twins.