This strain is homozygous for Pde6brd1, which causes early-onset retinal degeneration.
This strain carries a pair of linked retroviral insertions that occurred simultaneously on Chr 18 in a MEV substrain related to MEV/1Ty. Homozygosity for one of the insertions causes a juvenile lethal wasting condition that results in death at approximately 16 days of age. The proviruses rarely recombine; during development of this incipient congenic strain, a mouse was never found to have Emv21 in the absence of Emv22. There is no evidence that either of these viruses causes germline or somatic infection resulting in integration of new germline copies. (brs per personal communication from B. Taylor)
SWR/J was made congenic for a pair of linked retroviral insertions, identified as Emv21 and Emv22, in the laboratory of Dr. Benjamin Taylor by an initial cross to a strain closely related to MEV/1Ty followed by backcrossing of progeny at the N1 and subsequent generations to SWR/J for 6 generations (to N7). Siblings from the final backcross were then intercrossed to make the strain homozygous, after which it was maintained by brother-sister inbreeding.