TNFRSF25 and hematocrit: In Caucasians, HT is associated with HLA class II genes such as DR3, DR4, DR5, DQA1*0301, DQB1*0201 and DQB1*0301 [106-120] but not with the HLA-DP and HLA class I (HLA-A, HLA-B and HLA-C) genes [113,114,121].