A method for gender determination in newborn dark pigmented mice.

In many studies using mice, investigators must determine pups' gender at a very early postnatal stage. The gender of mouse pups is typically assessed by measuring the anogenital distance, which is greater in males than in females. This method, however, has proven to be difficult and not completely reliable. The authors describe a quick, easy and reliable method to establish the gender of pigmented mice. In male mice, a pigment spot on the scrotum is visible to the naked eye from the first day of life onwards, whereas female pups lack visible pigmentation in the anogenital region. In lightly pigmented or albino mice, the pigmentation is not obvious or not at all visible. The authors show that identifying this pigment spot is a more accurate and efficient method of determining pup gender compared with measurement of the anogenital distance. This 'spot on' method would therefore be a useful adjunct to conventional methods for determining the gender of pigmented neonatal mice.