The house mouse ( Mus musculus) is a small rodent, a mouse, one of the most numerous species of the genus Mus. As a wild animal, the house mouse mainly lives associated with humans, causing damage to cropsand stored food. The house mouse has been domesticated as the pet or fancy mouse, and as the laborartory mouse which is one of the most important model organisms in biology and medicine. It is by far the most commonly used genetically altered laboratory mammal. House mice have an adult body length ( nose to base of tail) of 7.5-10cm and a tail length of 5-10cm. The weight is typically 10-25 g. They vary in color from white to grey to light brown to black. They have short hair and a light belly. The ears and tail have little hair.