The U.S. Court of Appeals for the Ninth Circuit is preparing to rehear a key equal pay challenge asking whether employers can use salary history as a reason to pay a woman less than a man for the same work.

Last April, the en banc federal appellate court ruled employers could not use past salary as a defense. Prior salary standing alone is not a “factor other than sex” under the Equal Pay Act, said the court, and such a factor must be job-related.

The case, Yovino v. Rizo, was appealed to the U.S. Supreme Court. But in February, the high court did not reach the merits of the Equal Pay Act claim. Instead, the justices, in an unsigned opinion, vacated the Ninth Circuit ruling because, they said, the en banc court was wrong to count the late Judge Stephen Reinhardt's vote when it filed its ruling.