Mice homozygous for the severe kyphosis mutation have open eyelids at birth leading to injury and blindness, a progressive S-shaped kyphosis of the lumbar region of the spine, and a resultant shortened trunk and higher tail position. Small granulomas in muscle and brown fat are also found. Death usually occurs between 3 and 6 months of age and homozygotes do not breed.
The severe kyphosis mutation (sky) arose spontaneously in 1988 at The Jackson Laboratory in the DBA/2J inbred strain when that inbred was at generation F160. It was maintained on this background by sibling breeding for 40 generations then backcrossed twice to DBA/2J before again being maintained by sibling breeding. This strain reached generation F160+40N2F12 in 2008.