I was curious about how genetic could influence or contribute to drug addiction, so I did some research on this topic. Some people are more vulnerable to drugs and susceptible to drug addiction than others. Everyone could have access to addictive substances, but people who have the opportunity may not always take that risk. Biological differences can make a person more or less vulnerable to addiction. People with certain genes may have a more difficult time quitting when they start than others. When trying to quit, some people may experience severe symptoms of withdrawal than others. However a person's genetic makeup is not the only factor of addiction. Just because a person is susceptible to addiction, does not necessarily mean they will become addicted. Experience and a person's environment can play a large part in addiction. Susceptibility to addiction is the result of the interaction of many genes, not just a single gene. Research on the role of genetics in addiction linked the D2 dopamine receptor(DRD2 gene) to alcoholism. Studies that have been done on the brain showed that those with the A1 allele of the dopamine receptor, DRD2 have fewer dopamine receptors in the pleasure centers of the brain. Those individuals that become addicted to drugs that increase dopamine levels, become addicted easily because of that deficiency. Other studies have also shown that those with the A1 allele of the DRD2 gene are also addicted to cocaine, heroin, amphetamine and nicotine. These studies suggest that those with this genetic trait and more likely to have severe forms of addiction.