This program will support projects to identify and characterize factors associated with gender-affirming hormone therapy (GAHT) that may impact/contribute significantly to the prevention and treatment of HIV and/or co-infections with other sexually transmitted infections (STI)s in transgender individuals. For this funding opportunity GAHT is defined as a medical intervention, including but not limited to estrogen and/or testosterone treatment, that results in acquisition of secondary sex characteristics that align with an individual's gender identity. A growing body of data indicates that GAHT may impact the effectiveness and implementation of biomedical HIV prevention or treatment strategies and/or co-infections with other STIs. A number of factors in transgender populations present research challenges including variability in GAHT regimens, use of auxiliary drugs during GAHT care, access to participants, and access to existing cohorts, datasets, and samples. Consideration of appropriate controls, including cis-gender men and women is also a barrier. Recent studies demonstrating recruitment and retention of participants undergoing GAHT provide the opportunity to build on those studies and provide confidence that these critical studies are feasible.Letter of Intent Due Date(s) 30 days prior to the application due date