DNA fragments from cells that have died throughout the body not only appear in the bloodstream but also cross the kidney barrier into the urine. The relatively low molecular weight (150-200 bp) of this Transrenal DNA should be considered when deciding on methods of isolation and analysis. In particular, if polymerase chain reaction (PCR) is used for amplification and detection of specific sequences, then the reduction of amplicon size will significantly enhance sensitivity. Detection of DNA mutations is also made more difficult by the presence of a large excess of a wild-type allele. Using K-RAS mutations as an example, two ways around this problem--enriched PCR and stencil-aided mutation analysis-are described, based on selective pre-PCR elimination of wild-type sequences.