Forensic casework items sometimes contain less than 100 picograms (pg) of template DNA, which, assuming 3.5 pg of DNA per haploid cell, is equivalent to approximately 15 diploid or 30 haploid cells.  The presence of such low copy number (LCN) samples could be due to several factors including damaged or degraded DNA, oligospermic or aspermic perpetrators or from extended interval post coital samples, where sperm have been lost over time due to the effects of drainage or host cell metabolism. Trace biological evidence arising from casual handling of objects ("touch DNA") is increasingly being recovered from crime scenes.  Many of these "touch DNA" samples would also be classifiable as LCN samples due to the amount of DNA recovered.  Recovery of genetic profiles from LCN samples is difficult using standard STR (Short Tandem Repeats) methods and such attempts often result in total failure or recovery of partial profiles.  This is an expected outcome since commercial STR kits have been optimized to produce good quality, balanced profiles with 1 ng of DNA with 28-30 PCR cycles.  Hence special LCN methods, based upon increasing the PCR (Polymerase Chain Reaction) cycle number (ICN) to increase allelic signal intensity, have been developed to permit profile recovery from limited quantity samples.¹⁷ Interpretation of the data obtained from these LCN analyzed samples also requires novel considerations.¹⁷

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