Lateral femoral hernias in a line of FVB/NHsd mice: a new confounding lesion linked to genetic background?

Several strains of transgenic mice derived from an inbred FVB/NHsd colony developed large masses on 1 or both flanks. Although originally suspected to be a phenotypic anomaly related to genetic modifications, nontransgenic littermates subsequently were affected with equal frequency, inculpating the FVB/NHsd founder colony. The masses were subcutaneous, soft, and exophytic and appeared over the course of a few weeks. Female mice were affected more frequently than males. Gross examination revealed the masses to consist of uni- or bilateral hernias of variable size, occasionally containing small or large intestine (or both), cecum, mesenteric adipose tissue, male reproductive organs, and ureters. All hernial sacs pouched through the femoral triangle laterally to the femoral vessels and therefore were classified as lateral femoral hernias. Lateral femoral hernias have not previously been described in the veterinary literature and have never been described as background lesions in a strain of mice. Our findings suggest likely genetic drift in this strain of FVB/NHsd mice, causing a background lesion that confounded phenotypic analyses of transgenic mice derived from this strain.