The following paper is a description of a closely inbred strain of mice with a high incidence of mammary tumors which is being used in a number of widely scattered laboratories. These data are published in the hope that they will serve as a basis of comparison for the behavior of the strain under varying conditions of food, care, and climate.

Origin of the Stock: This stock originated in a single pair of mice obtained in 1909 by Dr. C. C. Little while he was working upon the inheritance of coat color in mice. The descendants of this pair were inbred brother-to-sister for twenty or more generations, care being taken to breed only those individuals which were the most vigorous. When the stock had reached this stage in its development an epidemic reduced it to a single pair. Some years later the colony obtained from this pair was again reduced to a very small number.

System of Breeding: During the entire period (1909–1932) the mice have been bred brother-to-sister, with the exception of a few occasions when daughters were back-crossed to their fathers or when sons were bred to their mothers.

No individual of the breeding colony has ever been out-crossed to any other strain of mice. Upon occasions when mice of this type were required for experimental crosses with other strains they were taken from the breeding stock but were not returned to it.

1 Accepted for publication, February 1933.

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