Breast cancer (BC) is a highly diverse complaint rather than sole disease consisting of several markers linked to typical features of tissues, medical assessment and reaction to treatment. Mutation in RAS/MEK/ERK and PI3K-AKT-mTOR pathway is involved in pathogenesis of BC. Application of pharmacogenomics will lead to individualization of therapy, which is totally contrast to nowadays clinical practice, in which drug's effects are studied on large group of patient regardless of their genetic based difference. The genetic differences in persons affect the therapeutic action and concentration of Tamoxifen in each individual. Therefore, it is, concluded to choose best drug regimen for each patient on individual basis and to circumvent the patient by toxic effect of drug.