NIH Public Access Policy 

You will now need to include the PubMed Central Reference Number (PMCID) when citing a work in a NIH report or grant application. To locate this number, use this unique search aid to locate articles in PubMed Central:

http://www.ncbi.nlm.nih.gov/entrez/pmc_cm.cgi

The National Library of Medicine has also created a PMID-PMCID converter. If an article is in PubMed and in PMC, it will have both a PubMed ID (PMID) and a PMC ID. Use this converter to translate one type of ID to the other.

http://www.ncbi.nlm.nih.gov/sites/pmctopmid

After locating the needed journal article in PubMed Central, look at the bottom of the reference. There, you will see a series of numbers and letters that start with "PMC". This string in the PubMed Reference Number (PMCID). Remember, this must be used when citing documents in any subsequent correspondence with NIH. See illustration below to find the PubMed Central Reference Number after locating a citation in PubMed Central.