The biggest difference between twin brothers Ethan and Aidan Dvash-Banks is impossible to see: One’s a U.S. citizen, and the other is not.

The State Department’s stance on the 16-month-old siblings prompted a federal lawsuit on behalf of their married gay parents, who charge the agency’s decision is discriminatory.

The sons of Andrew and Elad Dvash-Banks were each conceived with donor eggs and the sperm of one father, and delivered in Canada just minutes apart by a surrogate mom.

When the couple moved to Los Angeles, the boy sired by U.S. citizen Andrew was declared an official American. The child with Israeli citizen Elad’s DNA was denied the same status.

"How am I going to explain this to him when he grows up?" asked Elad, owner of a U.S. green card because of his marriage to Andrew.

“It's creating an issue from something that shouldn't be an issue.”

The citizenship controversy also threatens to divide the family, as Ethan entered the U.S. via a tourist visa that expired in December. The couple is currently trying to reapply.

The couple met a decade ago in Elad’s native Israel, where Andrew traveled as a student. Both men are named on the boy's birth certificates, according to the lawsuit.

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