- Apelin, the natural ligand of the orphan seven-transmembrane receptor APJ, inhibits human immunodeficiency virus type 1 entry. Cayabyab, M., Hinuma, S., Farzan, M., Choe, H., Fukusumi, S., Kitada, C., Nishizawa, N., Hosoya, M., Nishimura, O., Messele, T., Pollakis, G., Goudsmit, J., Fujino, M., Sodroski, J. J. Virol. (2000)