Nishimura, H., Agata, Y., Kawasaki, A., Sato, M., Imamura, S., Minato, N., ... & Honjo, T. (1996). Developmentally regulated expression of the PD-1 protein on the surface of double-negative (CD4-"CD8-") thymocytes. International immunology, 8(5), 773-780.
Salama, A. D., Chitnis, T., Imitola, J., Ansari, M. J. I., Akiba, H., Tushima, F., ... & Khoury, S. J. (2003). Critical role of the programmed death-1 (PD-1) pathway in regulation of experimental autoimmune encephalomyelitis. The Journal of experimental medicine, 198(1), 71-78.
Carreno, B. M., & Collins, M. (2002). The B7 family of ligands and its receptors: new pathways for costimulation and inhibition of immune responses. Annual review of immunology, 20(1), 29-53.