Several studies have linked marijuana use to increased risk for mental illnesses, including psychosis (schizophrenia), depression, and anxiety, but whether and to what extent it actually causes these conditions is not always easy to determine. The amount of drug used, the age at first use, and genetic vulnerability have all been shown to influence this relationship. The strongest evidence to date concerns the link between marijuana use and psychotic disorders in those with a preexisting genetic or other vulnerability. Recent research (see AKT1 Gene Variations and Psychosis) has found that marijuana users who carry a specific variant of the AKT1 gene, which codes for an enzyme that affects dopamine signaling in the striatum, are at increased risk of developing psychosis. The striatum is an area of the brain that becomes activated and flooded with dopamine when certain stimuli are present. One study found that the risk for those with this variant was seven times higher for daily marijuana users compared with infrequent- or non-users.