Unlocking the mysteries of breast cancer is in large part a matter of knowing which genes are involved in the disease, how and why they become flawed, and how those flaws cause healthy cells to transform into harmful ones. That’s why it’s such big news to learn that cancer researchers have recently identified a number of gene mutations that are furthering their understanding of these issues. Still more will be uncovered before long. So what are these mutations? Are we born with them? And if you carry them, what are the odds that they’ll lead to cancer? Here’s a breakdown of what’s going on and what it means for you, along with a little refresher course on genes.

Genes flawed from the start

Human genes occur in pairs—one copy inherited from each parent. Sometimes one or both copies of particular genes are mutated (flawed) in the body from the start—the so-called “inherited” gene mutations—predisposing us to diseases like breast cancer. One mutated copy of a gene in a cell may not be enough to cause disease. But if the other copy is disabled, or if enough mutations accumulate during cell division, that cell starts down the road to becoming cancerous. Scientists long suspected that some cases of breast cancer might be hereditary, but it wasn’t until the groundbreaking discoveries of the BRCA1 and BRCA2 genes in the mid-’90s that the theory was proven. Further studies pinpointed certain groups of women more likely to carry the BRCA mutations than the rest of the population.

Though the BRCA mutations are the most well-known inherited breast cancer gene mutations, scientists have identified inherited mutations and variations in a number of other genes that are implicated in breast cancer. Many of these interfere with the tumor-suppressor genes that normally keep cancer from developing. Others block the repair of damaged DNA within a cell, or promote the growth and proliferation of malignant cells in other ways. Being born with certain mutations can make a woman twice as likely to develop breast cancer in her lifetime as a woman with normal copies of those genes.

Genes that change with time

Unfortunately, having no family history of breast cancer doesn’t mean a woman is out of the woods, risk-wise. The majority of breast cancer cases are caused by noninherited, or “somatic,” gene mutations that occur over time. Mistakes in these genes can accumulate over decades, and are thought to be triggered by age, diet, hormones and other environmental influences. Breast cancer arises from the accumulation of multiple mutations. As we age, more and more chinks in the DNA structure develop, which helps explain why breast cancer is more common in older women.

One of the most significant somatic gene mutations is TP53, which was discovered in 1990. It’s a relatively common mutation—it can be found in as many as 20 to 40 percent of breast cancers—and a dangerous one, at that. Women with the TP53 mutation tend to develop tumors that resist treatment.

Getting a handle on this larger group of somatic gene mutations is an enormous task, but simply identifying them can help influence how women are treated. For instance, a mutation of the Her-2/neu gene causes a woman’s cells to produce too much of a particular protein. Herceptin—and the new drug Tykerb—were created specifically to block the production of the protein, each in its own way.

The Penetrance Factor

A crucial fact to consider when looking at a gene is its penetrance factor. This refers to your chances of actually developing cancer if you are carrying an abnormal copy of the gene.

A gene mutation is considered strong, or “high-penetrance,” if it greatly increases the odds of disease. Although the BRCA mutations are relatively rare—less than 1 percent of the population carries them—their penetrance level is quite high. Affected women may have as much as an 80 percent lifetime risk of developing breast cancer, as well as an elevated risk—20 to 60 percent—of developing ovarian cancer.

On the other end of the spectrum, a “low-penetrance” mutation is far less likely to lead to disease. For that reason, it’s not currently recommended that women undergo screening tests to detect low-penetrance gene mutations, the way they do if they are at particular risk of carrying a BRCA mutation. “There has to be at least a decent magnitude of risk to make it clinically useful, or there has to be a specific intervention,” says Jeffrey N. Weitzel, M.D., director of the Department of Clinical Cancer Genetics at City of Hope, a comprehensive cancer research and treatment center in Duarte, California.