Transferrin receptor 1 (TfR1) is the principle cell-surface receptor of MACV, JUNV, GTOV, and SABV [17, 18] , and a major determinant of host adaptation. However, studies on receptor use and cellular tropism suggest that the non-pathogenic Clade B viruses Amapari (AMPV) and Tacaribe (TCRV) can enter human cells in a human TfR1-independent manner [18, 19, 20] . Nevertheless, AMPV and TCRV use TfR1 orthologs of their principal host animals to infect nonhuman cells [21] . These studies reveal a complex pattern of receptor use for New World arenaviruses, suggesting the existence of additional receptor molecules, and a possible relationship between receptor use and disease potential.