A transplantable tumor, known as dBrD, gave very uniform results in a series of mice of known pedigree for a period of about a year.

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We may say that the reaction potential existing between the tumor and individuals inoculated was the same during this first part of the experiment. This tumor will grow in a mouse progressively provided there be present simultaneously at least from six to eight independently segregating genetic factors (genes) in the fundamental make-up of the host derived from the zygote that gave rise to the individual. In the course of routine transplantation of this tumor, there was a sudden change in this hitherto constant reaction potential resulting in at least three types of transplantable tumors—new types as far as their physiological activity and reaction potential are concerned. From a single original transplantable mass, I have therefore obtained four masses that remain true to type at least for some months of continued experimentation. These four types are (1) the original tumor, dBrD, still giving a six to eight Mendelian factor ratio, (2) dBrDm giving a two factor ratio, (3) dBrDBl showing another two factor ratio, and (4) dBrDBs giving a one factor ratio. The number of factors involved in the transplantability for the several tumors investigated are given in Table III.

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