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.