In the NBC television series Heroes, the superhuman abilities of the "Evolved Humans" (colloquially known as "EVOs") are the result a special gene complex involving two genes, each of which has two forms, or alleles. To be an EVO, a person must have at least one of each of the dominant alleles. The special gene complex creates a special protein that interacts with stress hormones, behavior-related neurotransmitters, immune system messengers, and other biochemical characteristics to produce chemical signals that modify the expression of susceptible genes. The special protein is a type of "genetic enhancer."