Three cell lines were chosen for further exploration: Vero E6, a Cercopithecus aethiops kidney epithelial cell line, expressing hACE-2, TMPRSS2, and AXL but not NRP-1; A549, a human alveolar epithelial cell line, expressing AXL, NRP-1, and TMPRSS2 but not hACE-2; and SCC7, a mouse skin cancer cell line, with no detectable hACE-2, AXL, NRP-1 or TMPRSS2 expression (Fig. 5a). The gene discussed is AXL; the disease is skin cancer.