A 38-year-old man has been HIV-positive since the age of 26 but has refused antiretroviral prophylaxis. His CD4 cell count, however, is still within normal limits, and the virus is undetectable in the blood. Genetic investigations clarify this apparent puzzle, demonstrating that this subject is homozygous for a mutation affecting a chemokine receptor necessary for HIV entry into the cell. Which of the following receptors is most likely involved?