Dark-haired HIV-negative men and women in San Francisco are being enlisted in a study that examines whether pre-exposure prophylaxis (PrEP) prevents HIV transmission, the San Francisco Chronicle reports.

According to the article, researchers are specifically enlisting volunteers with dark hair in the so-called “Strand Study” because drug molecules bind to hair pigment. Since dark hair has higher pigment levels, drugs are more likely to bind to it, making them easier to study.

The idea behind PrEP is that taking HIV meds before an exposure will prevent an infection. The study will require 24 volunteers to take various dosing regimens of tenofovir—a common HIV med that is currently being used in global PrEP studies—for six weeks. Researchers plan to begin the study in the next few months.

“Currently, we don’t have a truly accurate measure of how well people are taking their medications and how well they process drugs,” said Albert Liu, MD, director of HIV Prevention Intervention Studies for San Francisco’s Health Department. Liu clarified that binding quality has nothing to do with drug absorption; people with light hair metabolize drugs the same as their dark-haired counterparts.

The National Institute of Mental Health funds the study, which is conducted by researchers at the University of California in San Francisco and the city’s health department.