It is 100% illegal for an employer to refuse to hire someone because of race, color, religion, sex (including gender identity, sexual orientation, and pregnancy), national origin, age (40 or older), disability, or genetic information. However, a few negative and risky things can come up in a background check, which means you can choose not to hire someone because of their background. For example, you should not hire someone with a bad driving record to work for a taxi or other driving service.