http://www.sciencedaily.com/releases/2015/02/150213112317.htm

Research is newer, because transgenderism is newly accepted. As I stated previously, the trans part of the LGBT is decades behind the LG part. So, there isn't the same breadth of information, but given the fact that we've seen transgenderism pop up throughout history, I'm confident isn't inferring that we'll see a similar sort of biological basis for transgenderism that we see for sexual orientation.