The most common type of immunodeficiency disorders is severe combined immunodeficiency syndrome (SCID) followed by unspecified immunodeficiency (nonspecific changes of CD3, CD4, and CD8 count in peripheral blood flowcytometry; [International Statistical Classification of Diseases and Related Health Problems 10th Revision: D84.9]),[16] chronic granulomatous disease, HIV, and histiocytosis, respectively. The gene discussed is CD4; the disease is severe combined immunodeficiency.