LEAVENWORTH, Kan. (AP) – As the trial of a transsexual accused of lying on a marriage license form opened Nov. 15, a medical expert testified that sex and gender can sometimes be difficult to define.

“The one we commonly use, which is what we see at birth, the external genitals, is just one of the possible definitions,” Dr. Eric Vilain told The Associated Press after court adjourned on opening day in the trial of Sandy Clarissa Gast.

Gast, 48, was born Edward Gast and underwent sex change surgery in October after living for several years as a woman. She had planned to marry George “Georgi” Somers, who is a man but lives as a woman.

Gast is charged in Leavenworth County with false swearing, a misdemeanor carrying a maximum fine of $500. Prosecutors said she lied when she signed as a woman on the marriage license application, filed in February.

Vilain, head of medical genetics at UCLA, said he had not examined Gast or even met her before the trial. But he said it is not unheard of for someone to appear to be one sex at birth but carry characteristics of the other sex.

“You can define sex with chromosomes, with gonads – whether someone has testicles or ovaries – with hormone levels … with internal genitalia you don’t see from outside,” Vilain said. “There are also differences in brain structures between men and women, and that’s another way to define sex, if you will. In a lot of people, all these various ways to define sex are congruent with each other. But in a relatively large number of people, there are discrepancies.”

Gast, who was arrested in March after Somers’ daughter emailed prosecutors in an effort to stop the planned marriage, is being defended by the American Civil Liberties Union.

The state rested its case after presenting one witness: the court clerk who took Gast’s application for a marriage license.

Leavenworth County attorney Frank Kohl contends Gast is still legally male despite the sex change surgery, and he has case law on his side.

However, said defense attorney Pedro Irigonegaray, the judge in this case is not being asked to decide whether Gast and Somers have the right to marry.

“The issue is not whether the state may deny a transgender the right to marry, but whether a transgender, when she signs as a woman, is telling the truth,” Irigonegaray said. “We think Gardiner should be overturned, but in this case you don’t have to overturn Gardiner to reach the decision that when Miss Gast signed as a bride, she had no specific intent to commit a crime.”