Beginning at approximately 10 days of age, pi/pi mice first display backing or crab-like movements and by approximately 14 days of age pirouetting is observed. This pirouetting is a spinning movement that involves swinging the head sharply to one side, either left or right, placing the inner rear foot (with the toes facing outward) close to the outer rear foot which then is used to push the mouse in a tight circling motion. This spinning persists for relatively long periods and cyclostat assessment suggests that these mutants do not develop rotational dizziness. Adult pi/pi mice can not swim on the surface of water or in a coordinated manner and are deaf at 21 to 30 days of age as determined by auditory brainstem response and a failure to respond to a variety of sounds. Although the organ of Corti develops normally, degeneration is found by the second week of life. The tectorial membrane is thickened at 12 days of age and subsequently the hair cells degenerate and the supporting cells lose form. By four weeks of age degeneration of the spiral ganglion is found. This is followed by degeneration of the saccular macula by approximately 6 weeks of age and degeneration of the stria vascularis after that. The macula of the utriculus remains unaffected. pi homozygotes differ from an array of similar labyrinth degeneration mutants in that the cristae ampullares degenerate, albeit after eight months of age. (Woolley and Dickie, 1945; Dickie and Woolley, 1946; Deol 1956; Beyer et al., 2000.)

These mice carry a spontaneous mutation at the Grxcr1 locus characterized by abnormal mobility with a circling motion.

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