At approximately two months of age, flying squirrel homozygotes stiffen throughout their trunk and limbs, particularly on the ventral side of the body, and when picked up take on a rigid spread out posture. As a result they walk with a slight stagger. Homozygotes are not fertile but do live to adulthood.
The flying squirrel mutation arose spontaneously in the KK.Cg-Ay background and in 1998 in vitro fertilization was performed using a homozygous fsq male and nonagouti (a/a) females from the KK/Cg-Ay/J strain. The resultant offspring were intercrossed to generate this nonagouti strain, which has been maintained by breeding hosts of fsq homozygous ovarian transplants with +/? sibling males and then intercrossing the heterozygous offspring.