Other immunoregulatory genes are involved in the development of HT, including the single nucleotide polymorphisms (SNPs) in cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), protein tyrosine phosphatase non-receptor type 22 (PTPN22), and CD40 [2,4,6,7,18,19,20]. Here, PTPN22 is linked to hematocrit.