A 2004 study carried out at the University of Colorado found that around 15 per cent of Caucasians have a genetic variant, known as the G-variant, that makes ethanol behave more like an opioid drug, such as morphine, with a stronger than normal effect on mood and behaviour. This variant seems randomly distributed among the population: it emerged through mutation, although the factors affecting its selection remain unknown since, like all genes, it does not operate in isolation. . . . The Colorado study tested the DNA of moderate-to-heavy drinking students to determine whether they had the G-variant gene. They were divided into two groups accordingly, before having alcohol injected directly into the bloodstream (to eliminate differences in absorption rate). Those with the G-variant produced a slightly different version of what is known as the mu-opioid protein, which elicits a stronger response in the brain. As a result they reported stronger feelings of happiness and elation after their shot of alcohol. This initial euphoria is usually followed by a longer state of relaxation, lasting several hours.