The NIH Public Access Policy says “When citing their NIH-funded articles in NIH applications, proposals or progress reports, authors must include the PubMed Central reference number for each article” and the FAQ provides some examples of this:
Examples:
Varmus H, Klausner R, Zerhouni E, Acharya T, Daar A, Singer P. 2003. PUBLIC HEALTH: Grand Challenges in Global Health. Science 302(5644): 398-399. PMCID: 243493
Zerhouni, EA. (2003) A New Vision for the National Institutes of Health. Journal of Biomedicine and Biotechnology (3), 159-160. PMCID: 400215