LDLR and infection: We now recognize four receptor proteins that are necessary for the host cell infection by ALV subgroups: Tva protein related to the low-density lipoprotein receptors (LDLR) [2,3], Tvb protein belonging to the tumor necrosis factor receptor family [4,5,6], Tvc protein similar to mammalian butyrophilins [7], and Tvj, which was identified as a Na+/H+ exchanger 1 [8].