In the past few decades, many studies have demonstrated that NK cell activity is much lower in individuals with different types of cancers. NK cell activity tends to be lower in healthy individuals whose parents, siblings or other family members have cancer. This suggests that low NK cell activity is an important risk factor for the development of a variety of cancers. This theory has been confirmed in a study that followed more than 3000 people for a span of 11 years, and in which low NK cell activity was used to predict the risk of cancer. People with a genetic predisposition to low NK cell activity may be at higher risk for developing cancer.