These classes include the ecotropic viruses, limited to rodents (mCAT1 receptor), xenotropic viruses (excluded from infection of inbred mice [22]; Xpr1 phosphate exporter receptor), and polytropic/mixed polytropic viruses [24], infecting mouse and nonrodent species [22, 23] using Xpr1 as their receptor [25]. The gene discussed is XPR1; the disease is infection.