The FBI uses STR (short tandem repeat) analysis to identify criminals, and the FBI stores the information in a database. How many regions, or loci, of DNA are used for STR analysis?

The answer is thirteen.

STR stands for short tandem repeats. In this analysis, highly polymorphic regions with short tandem repeats are used. In FBI investigation, thirteen regions are analyzed and compared. The chance for two people to have the exact same thirteen regions is almost impossible (1 : 1 billion or less). This way a difference between two DNA profiles could be assessed.