PubMed Central

PubMed Central (PMC) is a free digital archive of articles, accessible to anyone from anywhere via a basic web browser. The full text of all PubMed Central articles is free to read, with varying provisions for reuse. Submissions into PMC undergo an indexing and formatting procedure which results in enhanced metadata, medical ontology, and unique identifiers which all enrich the XML structured data for each article on deposit.

PubMed Central use a unique identifier called PMCID to index articles.

Example
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2586059/

The PMCID number for this study is PMC2586059.

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 * Wikipedia - PubMed Central