Combined immunodeficiency due to OX40 deficiency is a rare combined T and B cell immunodeficiency characterized by susceptibility to develop an aggressive, childhood-onset, disseminated, cutaneous and systemic Kaposi sarcoma. The disease is combined immunodeficiency due to OX40 deficiency (Monarch Disease Ontology term MONDO_0014268). Also known as: combined immunodeficiency with childhood-onset Kaposi sarcoma, combined immunodeficiency with impaired immunity to HHV-8, combined immunodeficiency with impaired immunity to human herpes virus 8, immunodeficiency type 16.