Hepatitis B testing was done at all 3 study locations: RDS calculations revealed that Mexico had 88.3% and Texas 48.6% of IDUs positive for anti-HBc, revealing individuals who were not susceptible to infection (i.e., immune due to natural infection whether acute infection or chronically infected), compared with New Mexico's crude calculation of 59.6%. This evidence concerns the gene PPIB and hepatitis A virus infection.