How is it done? 

This is a very simple test. 

First, a small blood sample is collected in a hospital or clinic and sent to a specialized laboratory.

In the laboratory the test can be done through different methods.

Most genetic tests look for changes in the normal DNA sequence of the gene that is being tested 

To learn how mutations are linked with cancer Biology of cancer . 

Other genetic tests do not look at the DNA, but instead at the proteins it produces. They analyze the length or the functionality of the protein, looking for abnormalities. 

In breast cancer, most tests look for changes in the DNA of the BRCA1 and the BRCA2 genes (BRCA = BReast CAncer genes), which are located, respectively, in chromosome 17 and 13.

Sometimes, in very particular cases, testing can also include the genes PTEN and TP53, which are associated with the Cowden and the Li-Fraumeni breast cancer syndromes. These are very rare though, accounting for less than 1% of all the hereditary breast cancers.

Due to their complexity, test results can take a few months.