NOS3 and Hypertension resistant to conventional therapy: The handful of studies that have specifically investigated resistant hypertension have mostly focused on candidate genes: β and γ subunits of the epithelial sodium channel (ENaC) [17], cytochrome P450 gene (CYP3A5) [18], glutathione S-transferase mu type 1 gene (GSTM1) [16], and endothelial nitric oxide synthase gene (NOS3) [19, 20].