CODIS, the combined DNA index system, is a data bank consisting of the DNA from violent offenders. When a violent crime like rape, murder or assault is committed, and evidence from the scene, like blood, skin cells or semen is available, a DNA profile from that evidence is produced.
It is compared to the DNA in the CODIS data base. If no match is found, it is virtually impossible to find the perpetrator without other incriminating evidence or a suspect.
An interesting case occurred in N.Y.C., where a serial rapist terrorized city neighborhoods over a period of years. Police knew it was the same person committing each crime because the DNA from each case matched. But there was no match from the CODIS data base. It was not until a suspect was found from an eye witness identification that the DNA profile, taken from the suspect matched the evidentiary DNA and the suspect was indicted.
