Lesbian, gay, bisexual, trans, and intersex (LGBTI) persons are at particular risk of being targeted in situations of violence because of their gender or sexual preference. Discrimination against LGBTI persons could be based on sexual orientation (to whom someone is attracted); gender identity (how people define themselves, irrespective of biological sex), gender expression (how gender is expressed, through clothing, hair, or make-up), or sex characteristics (for example, genitals, chromosomes, reproductive organs, or hormone levels). The UN Special Rapporteur on extrajudicial, summary or arbitrary executions has observed that LGBTI persons are especially vulnerable to extrajudicial killings. The prohibition on executing LGBTI persons on the basis of their gender or sexual identity is not only a customary rule, but it is also a peremptory norm of international law.