For many years we have maintained in this laboratory a number of inbred strains of mice including the so-called high-mammary-tumor strains C3H, C3H-AVY, DD, and A, all of which have the mammary tumor virus (MTV) and two low-mammary-tumor strains C57BL and BALB/c in which MTV has not been demonstrated. There are certain differences among the high-mammarytumor strains. C3H-AVY is so designated because it bears the AvY mutation that arose in C3H and has been shown to increase suceptibility to a number of tumors including mammary gland tumors. DD can be expected to differ from the other strains since it came from Japan and is unrelated to the other strains. Strain A is unusual in that, while the breeding females have a relatively high incidence of mammary tumors, the incidence in the virgins is very low. Even the lines of MTV carried by these strains are somewhat different. There is also a great difference in the two low-tumor strains. C57BL not only does not have the MTV but is genetically very resistant to it, whereas BALB/c is very susceptible to MTV.Various combinations of these mouse strains and the lines of MTV have been made by transferring the lines of virus to mouse strains other than their original host strain through foster nursing in order to observe how this might alter the development and incidence of mammary tumors. In addition, the milk-transmitted MTV

Mammary tumorigenesis in C3H-A(vy)fB female mice is not due to milk-borne mammary tumor virus but to factors transmitted at conception. Prominent among these is the A(vy) gene which may increase the virulence or transmissibility of a variant of mammary tumor virus vertically transmitted by either parent, or may increase the tumorigenic response of the mammary tissue. These factors together with the influence of hormones of pregnancy resulted in the high incidence observed.

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