Compared to those of Caucasians, the plasma levels of atazanavir (ATV) may be higher in Koreans with the same dosage regimen(s). If so, even unboosted ATV could be used with tenofovir DF (TDF) which lowers the concentration of ATV. The investigators plan to investigate the pharmacokinetic features of ATV with or without TDF in healthy Korean and Caucasian volunteers and compare the ethnic differences.

Inclusion Criteria:

• Men aged 20 to 55 years old, with a weight more than 50 kg and with appropriate body mass index (BMI) values within 19-28 kg/m2.
• Agreed voluntarily to participate to the study and comply with the study protocol with written permission.

Exclusion Criteria:

• Has history of liver, kidney, respiratory, musculoskeletal, endocrinologic, neuropsychiatric, hemato-oncologic, or cardiovascular problem(s).
• Has history of hypersensitivity or clinically significant adverse drug reaction(s) to the study drugs, same class of the study drugs, or other drugs including aspirin and antibiotics.
• Drinks excessive caffeinated beverages (caffeine >10 units/day), alcohol beverages (alcohol >21 units/week) or smokes excessively (>20 cigarettes/day) or has history of alcoholism.
• Has been excluded by the researchers due to abnormal findings in screening EKG and/or blood tests