Although the full nature of these viral load-independent parameters is as yet unknown, their importance in pathogenesis is inferred from (i) prior findings showing that the viral load does not explain the full extent of variability in AIDS progression rates [39] or rate of CD4 cell decline [44], [45], and (ii) findings in non-human primates who are naturally infected with simian immunodeficiency virus, as these animals exhibit a non-progressive disease despite high viral loads [58]. The gene discussed is CD4; the disease is AIDS.