Biological sex is the sole basis for defining the words “male,” “female,” “father,” and “mother,” according to new guidance published Wednesday afternoon by the Department of Health and Human Services.

In line with President Donald Trump’s Day One executive order defining male and female on an individual’s reproductive cells, or gametes, HHS issued guidance defining sex as “a person’s immutable biological classification as either male or female” based upon the respective production of sperm or eggs.