HJV and hemochromatosis type 1: On the other hand, four types (types 1, 2, 3, and 4) of hemochromatosis have been genetically classified on the basis of mutations in five genes (HFE, human antimicrobial peptide [HAMP], hemojuvelin [HJV], transferrin receptor 2 [TFR2], and SLC40A1).