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Automating the Differential Digestion Method in the Analysis of Sexual Assault Cases Using Selective Degradation

NCJ Number
Helena G. Wong; Jennifer S. Mihalovich; George Sensabaugh
Date Published
April 2013
48 pages
This project developed and tested an automated differential digestion process that uses selective degradation with DNase 1 in order to remove non-sperm DNA from a mixed sample of evidence in a sexual assault case, thus saving time and labor.
The protocol uses 96-well plates and incorporates microscope slide preparations, resulting in high efficiency and high throughput. Although initial evaluations of the DNase process produced lower DNA yield and inferior STR DNA typing quality compared to the conventional differential digestion. This was corrected by optimizing the concentrations of the divalent ions magnesium and calcium, along with the quantity of DNase used. This modification increased both the DNA yield and the quality of the STR DNA typing results. The protocol was tested on samples stored for extended time periods (up to 60 years) and on samples exposed to multiple freeze/thaw cycles or heat and humidity. All samples subjected to the new protocol produced results similar to samples processed by the conventional differential digestion method. The result was the production of sperm fractions with no or minimal epithelial cell carry-over. 7 tables, 29 figures, and 22 references