The invention relates to methods for preventing cancer in a KRAS-variant subject which include administering to the KRAS-variant subject an amount of estrogen effective to reduce the risk of developing cancer. In another aspect, the invention further relates to methods for treating cancer in a KRAS-variant subject, which include gradually decreasing estrogen exposure in the KRAS-variant subject to reduce the risk of aggressive tumor growth. In another aspect, the invention relates to a method of predicting an increased risk of developing a second, independent breast cancer in a subject. The method can include detecting a single nucleotide polymorphism (SNP) at position 4 of the let-7 complementary site 6 of KRAS in a patient sample wherein the presence of said SNP indicates an increased risk of developing a second, independent cancer in said subject.
