Another unexpected finding is that hair color may affect pain tolerance. According to a 2009 study published in the Journal of the American Dental Association, redheads are more sensitive to pain and may require more anaesthetic for dental treatments. Darker hair colors also seem to be associated with higher levels of pain-killing hormones in the body. The study concluded that redheads might benefit from reduced doses of anaesthesia during procedures. In another study conducted at McGill University in Canada, it was found that women with dark hair colors required less anesthesia than those with light hair colors. The researchers suggested that this may be because females with darker hair colors have more opioid receptors, which are proteins on cell surfaces that bind with morphine-like substances such as enkephalins or endorphins. This binding causes the cells to release neurotransmitters, which cause pain signals to be blocked. It is not known exactly how many people are redhead, but estimates range from 1% to 6% of the population. People who are redheaded have copies of the RHCG gene called alleles that differ by one or two letters at position 213 of the gene code. These genetic differences lead to different levels of hair coloring agents being processed through skin cells containing the RHD gene. This process results in people who are redheaded having darker hair colors than others who do not have this genetic makeup.