In those prone to or with hereditary form of breast cancer there is mutation or flaw in the genes called BRCA1 and BRCA2. BRCA1 is a gene that produces a type of proteins called breast cancer type 1 susceptibility protein. They are normally expressed in the cells of breast and some other organs. Whenever there is a damage to DNA of the cells in these organs BRCA1 immediately repairs this DNA damage or if it cannot be repaired it kills the cells. Thus cancerous tumors are killed at the very early stage itself. The function of BRCA2 gene too is more or less similar though the molecular structure is different. BRCA2 stands for Breast Cancer 2 susceptibility protein. It's main function is to repair the chromosomal damage and fix the breaks in the DNA structure. Thus it can be seen that proper functioning of these two genes are very important to protect us against breast cancer. Whenever there is a flaw in these two genes which is mainly due to hereditary reasons the person is at a huge risk of contracting breast cancer. 

In breast cancer tumors with BRCA mutation drugs that inhibit Poly ADP ribose polymerase (PARP) inhibitors cause double strand break in the cells. Because of BRCA deficiency these cannot be repaired. This leads to the death of cancerous cells. Normal cells do not replicate the way cancer cells do so the normal cells are not affected by PARP inhibitors.

The same principle is applied by scientists to use PARP inhibitors to treat tumors which do not have this genetic flaw but by artificially inducing such a genetic flaw. CDK1 or Cyclin dependent kinase 1 is a protein that is very vital in maintaining normal cell cycle process. It repairs any damage to the DNA of the cells. By blocking the CDK1 molecule the tumors are deprived of the DNA repair mechanism. When the subject is given PARP inhibitors the cancerous tumor was successfully shrunk. Scientists have effectively treated lung cancer in rats using this method.