Diabetic prone BB rats. The Biobreeding rats (BB) spontaneously developed autoimmune diabetes mellitus were found in a closed colony of outbred WI (Wistar) rats at the Bio-Breeding Laboratories, Ottawa, Ontario in 1974. In 1977, Butler et al. began inbreeding BB rats at the University of Massachusetts Medical Center
(laboratory code Wor) with 300 breeders purchased from the Bio-Breeding Laboratories. In 1978, during inbreding, pathogen-free rodent barrier system was introduced and and 2 strains were produced: disease prone (DP) and disease resistant (DR) by selected progenies were diabetic (DP) or remained diabtes free (DR).