the variant genotype more frequently, but had an allele distribution equivalent to cisgender male controls, unlike the cisgender female controls. The paper concluded that the loss of a female-specific CYP17 T -34C allele distribution pattern is associated with transmasculinity. Gender incongruence among twins 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. The study found that one third of identical twin pairs in the sample were both transgender: 13 of 39 (33%) monozygotic or identical pairs of assigned males and 8 of 35 (22.8%) pairs of assigned females. Among dizygotic or genetically non-identical twin pairs, there was only 1 of 38 (2.6%) pairs where both twins were trans. The significant percent of identical twin pairs in which both twins are trans and the virtual absence of dizygotic twins (raised in the same family at the same time) in which both were trans would provide evidence that transgender identity is significantly influenced by genetics if both sets were raised in different families. Brain structure General Transgender brain studies, especially those on trans women attracted to women (gynephilic), and those on trans men attracted to men (androphilic), are limited, as they include only a small number of tested individuals. Several studies have found a correlation between gender identity and brain structure. A first-of-its-kind study by Zhou et al. (1995) found that in the bed nucleus of the stria terminalis (BSTc), a region of the brain known for sex and anxiety responses (and which is affected by prenatal androgens), cadavers of six trans women had female-normal BSTc size, similar to the study's cadavers of cisgender women. While the trans women had undergone hormone therapy, and all but one had undergone sex reassignment surgery, this was accounted for by including cadavers of non-trans female and male controls who, for a variety of medical reasons, had experienced hormone reversal. The controls still had sizes typical for their sex. No relationship to sexual orientation was found. In a follow-up study, Kruijver et al. (2000) looked at the number of neurons in BSTc instead of volumes. They found the same results as Zhou et al. (1995), but with even more dramatic differences. One transfeminine subject who had never received hormone therapy was also included, and nonetheless matched up