Although the six host cell lines were derived from alternate tissue types (e.g., lung, kidney, uterus) which may vary in their TfR expression levels and/or the amounts of virus they produce, all were susceptible to infection (consistent with the pantropic nature of these viruses) and thus provide the most appropriate in vitro models for these adaptation studies until a tissue type-specific cell line is developed for multiple different carnivore species. The gene discussed is TFRC; the disease is infection.