The sequencing of the human genome is only the first step. The next step is to determine the function of these genes and in particular, how alterations in specific genes lead to major human disorders. Many laboratories are now focusing on identifying and characterizing single nucleotide polymorphisms (SNPs), to determine which correlate in frequency with certain population groups who may be particularly susceptible to certain diseases. The mu opioid receptor (MOR), which mediates the clinically important analgesic effects of drugs like morphine as well as the euphoria sought by heroin abusers, exhibits several dozen polymorphisms. Several of these are associated with altered receptor function and individuals at risk for drug abuse.