Clasper is a spontaneous, recessive mutation phenotypically apparent by 14 days of age and neither deteriorating nor improving thereafter. This mutation causes whole body tremor followed by clasping of the paws when the mouse is suspended by the tail. Homozygous males and females are viable and fertile but smaller in size than normal. Homozygotes often have a hobbling gait resulting from the tendency to clasp the rear paws and hold them close to the body. (Sweet HO, 1985).
The clasper mutation arose in 1980 on the BALB/cBy background and was maintained on this background until it was frozen in 1990. It was frozen by breeding C57BL/6J females to heterozygous males thus all frozen embryos are currently +/?.