Mice homozygous for the fidget spontaneous mutation (Fignfi) toss their heads from side to side and tend to run in circles. Homozygous fidgit mice are hypersensitive to sound in early life, but later lose hearing ability. Corneal lesions are common, and hind foot polydactylism occurs. The bony labyrinth is quite defective. Other abnormalities including small eyes, absence of lachrymal glands, dislocation of the hip, and displaced parafloccular lobes of the cerebellum. Viability of homozygotes is usually less than normal and fertility is poor. The STOCK is also carrying the stubby spontaneous mutation (stb). Homozygous stubby mice are recognizable at 4 or 5 days by a domed head and thick short tail. Adult stubby mice have shorter heads, bodies, and legs compared to wildtype mice. Most females are fertile but males do not breed. Male mice do produce normal numbers of motile sperm. Cartilage histology is within normal limits, but chondrogenesis stops earlier than in normal animals.

These mice carry a spontaneous mutation at the Fign locus characterized by head tossing and circular running. Fidget mice also have insufficient growth of the retinal neural epithelium resulting in small eyes, and skeletal abnormalities, including pelvic girdle dysgenesis, skull bone fusions and polydactyly.

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