There have been two main types of forensic DNA testing. They are often called, RFLP and PCR based testing, although these terms are not very descriptive. Generally, RFLP testing requires larger amounts of DNA and the DNA must be undegraded. Crime-scene evidence that is old or that is present in small amounts is often unsuitable for RFLP testing. Warm moist conditions may accelerate DNA degradation rendering it unsuitable for RFLP in a relatively short period of time.
PCR-based testing often requires less DNA than RFLP testing and the DNA may be partially degraded, more so than is the case with RFLP. However, PCR still has sample size and degradation limitations that sometimes may be under-appreciated. PCR-based tests are also extremely sensitive to contaminating DNA at the crime scene and within the test laboratory. During PCR, contaminants may be amplified up to a billion times their original concentration. Contamination can influence PCR results, particularly in the absence of proper handling techniques and proper controls for contamination.PCR is less direct and somewhat more prone to error than RFLP. However, PCR has tended to replace RFLP in forensic testing primarily because PCR based tests are faster and more sensitive.