This congenic was developed by backcrossing virus resistant Japanese wild mice Mus musculus molossinus to BALB/c (Odaka, et al 1978 and 1981). Resistant mice were selected by spleen weight (virus-induced splenomegaly) and progeny testing results (Odaka, et al 1981). The virus used was the Friend isolate of murine leukemia virus. Several congenic lines were developed including an N6 congenic line designated C4W (from the wild mouse Fu), and an independently derived N6 congenic line described as Hz (from the wild mouse Hz-IV) (Odaka, et al 1978, Kozak, et al, 1984, Ikeda, et al 1985). The Hz-IV mice were originally tested by crossing to C57L and C57BL/6, it is not clear if the backcross to BALB/c began with these F1 mice or the original Hz-IV mice. Although the history is ambiguous, we believe Dr. Wallace Rowe at the NIAID Laboratory of Viral Disease received mice derived from Hz-IV from Dr. Odaka at the University of Tokyo. Dr. Morse obtained these mice from Dr. Rowe in 1983. The strain has been maintained by brother x sister matings since that time. The backcross generation is unknown, but is greater than N6. This strain was donated to The Jackson Laboratory in 2004.
