Interestingly, although eosinophils also express CCR5 [89], in vitro studies showed that only CXCR4-tropic HIV strains can give rise to productive infection [154] and some authors speculate that the inability of CCR5-tropic viruses to actively infect eosinophils may be due to the necessity of higher levels of CCR5-expression [155], as shown for CD4+ T cells [156]. The gene discussed is CD4; the disease is infection.