SummaryIt is suggested that the anomalies in the IS rat are controlled by one or several genic factor(s) unrelated to LG I, II, IV, V, and VI and the LGs containing Amy-l and Cs-lloci.In the IS strain of rat (Rattus norvegicus), almost all individuals express singly or jointly several vertebral anomalies, mainly of the lumbar vertebrae. The anomalies include complete and incomplete unsegmentation, wedged and butterfly vertebrae, and result in scoliosis, kyphosis or both in about 50% of the rats. So, the IS strain is considered to be a spontaneous animal model of human congenital kyphoscoliosis (Ishibashi, 1979).In mice, many vertebral anomalies, including those of tail vertebrae, have been reported as monogenic or multifactoral traits (Kalter, 1978). In contrast, in the rat such defects are rarely reported. The following seem to comprise the known mutants: Chubby (cb), incisorless (in), Griineberg's lethal (lg), hypodactylyluxate (Ix), osteopetrosis (op), stubby (sb), stub (st), and toothless (tl) (Robinson, 1979). In the IS rats the anomalies are quite different morphologically from these gene effects in mouse and rat, and on the basis of preliminary experiments they seem to be controlled by 2 or more factors.We report here on the type, location, and degree of the vertebral anomalies in the IS strain, and on the results of a classical segregation analysis, involving crosses of the IS strain with inbred and partly inbred strains with normal vertebrae. We are developing a BN congenic line with the vertebral anomalies of the IS strain by repeated crossing and intercrossing. In this paper, we report the results to the N3F3 generation.