Genetic variation within this KRAB-ZNF gene cluster has been associated with at least three medical conditions in humans: hemochromatosis (iron overload, the most prevalent genetic condition in Europeans), CD4:CD8 lymphocyte ratios, low production of CD8+ effector memory (TEM) and double negative (TDN) T-cells, and schizophrenia. This evidence concerns the gene CD4 and Tangier disease.