Following isolation, samples were quantified by spectrophotometry using a Thermo Nanodrop (Thermo Fisher Scientific, Waltham, MA). Our threshold for considering a sample acceptable was a minimum mass of 20 ug, yielding a minimum concentration of 20 ng/ul in 1000 ml. For samples with yields of 10–19.9 ug, the resulting concentrations were too low (<20 ng/ul) for subsequent experimental needs. Purity of the nucleic acid preparation is generally high for DNA isolated from saliva. Across all samples collected and isolated using Oragene tubes and reagents, the mean (SD) OD260/OD280 ratio was 1.85 (0.12). These samples were reprecipitated by addition of 0.1 volume of 3 M NaOAc (300 mM final concentration) and two volumes of 100% ethanol, incubation at −80°C for 30 min and centrifugation at 15,000 rpm for 5 min. Reprecipitated samples were resuspended in an appropriate volume of TE to yield a concentration >20 ng/ul based on the total amount of DNA reprecipitated. Individuals whose samples did not yield the minimum 10 ug were asked to provide a second saliva sample.
