The exact cause of Tourette's is unknown, but it is well established that both genetic and environmental factors are involved. Genetic epidemiology studies have shown that Tourette's is highly heritable, and 10 to 100 times more likely to be found among close family members than in the general population. The exact mode of inheritance is not known; no single gene has been identified, and hundreds of genes are likely involved. Genome-wide association studies were published in 2013 and 2015 in which no finding reached a threshold for significance. Twin studies show that 50 to 77% of identical twins share a TS diagnosis, while only 10 to 23% of fraternal twins do. But not everyone who inherits the genetic vulnerability will show symptoms. A few rare highly penetrant genetic mutations have been found that explain only a small number of cases in single families (the "SLITRK1, HDC", and "CNTNAP2" genes).