This retrospective case-control study is being conducted to estimate the sensitivity of the pharmacogenetic marker, HLA-B*5701 for hypersensitivity to abacavir (ABC HSR). The specificity of the marker, and its association with ABC HSR, as measured by odds ratios and 95% confidence intervals, will be evaluated as secondary endpoints. Cases will be defined in two ways - subjects who have clinically-suspected ABC HSR and a positive abacavir skin patch test reaction (CS-SPTPos) and subjects with clinically-suspected ABC HSR (CS-HSR), regardless of the results of skin patch testing. The study will include 40 CS-SPTPos Black cases matched with up to 200 abacavir-tolerant controls. In parallel, 40 CS-SPTPos White cases will be matched with up to 200 White controls. Some of the secondary analyses will use cases defined by clinical criteria alone (CS-HSR). Black and White subjects will be analyzed separately because of the differences in ABC HSR rates and in the carriage frequency of HLA-B*5701.