Some combination of genetic, biological and environmental factors is believed to cause ASDs. Researchers are exploring several genes which are believed to contribute to the development of these disorders as well as several brain regions that have been linked to the disorders. Abnormal brain development during the first months of life is being studied to determine if structural abnormalities, such as in the mirror neuron systems, may be caused by genetic and/or environmental factors. Researchers are also exploring the effects of genetic imprinting in which a gene’s expression is determined by which parent donates the gene copy. Certain neurotransmitters, such as serotonin, dopamine, and epinephrine, may also function abnormally. In some cases, scientists are exploring the possibility that a faulty immune response to a virus, elevated concentrations of proteins in the blood at birth, dysregulation of specific neuropeptides or a major stress during pregnancy may lead to the disorder.