Remains from about half of the approximately 2,800 victims have been identified, many through the analysis of DNA in tissue samples recovered from the site. Identification is difficult because of the damaged and degraded condition of the samples. Now that standard methods have revealed all they can, new techniques for analyzing very small fragments of DNA are being used to reexamine the genetic material from as many as 13,000 recovered bone samples.
One such technique is adapted from a NIST-developed method using short tandem repeats (STRs)--tiny segments of DNA containing two to five base pairs (the nitrogen compounds whose unique pairings make up the genetic code of an individual). By contrast, a complete human genome contains some 3 billion base pairs.
To perform the STR analysis, many copies are made of targeted DNA fragments using a method called the polymerase chain reaction (PCR). NIST designed new PCR primers--tools for targeting the amplification (copying) to a specific DNA segment--that reduce the size of the amplified regions by 100 base pairs or more. Smaller PCR products amplify better than large ones when genomic DNA is degraded.