The study authors suggested that the findings could lead to improved treatment for patients with schizophrenia and bipolar disorder, which affect about 3 percent of people worldwide.
Classifying patients based on their repeat arrays may help identify those most likely to respond to current calcium channel drugs, which so far have produced mixed results, Kingsley said.
He added that more research is needed to determine whether patients with a genetic variation of CACNA1C have too much or too little calcium channel activity.
The repeat arrays in the CACNA1C gene occur only in humans. Kingsley said that suggests the arrays may have given humans an evolutionary advantage, even if they increased the risk of conditions such as schizophrenia and bipolar disorder.