From the NLM Technical Bulletin:
On March 31, 2013 the National Library of Medicine (NLM) began use of RDA: Resource Description and Access for all original cataloging of modern material, replacing the Anglo-American Cataloguing Rules, 2nd edition (AACR2).
Bibliographic records distributed by NLM will continue to contain a mix of RDA and AACR2 records as NLM will accept AACR2 copy without upgrading these records to RDA. Any new authority records created by NLM will follow RDA guidelines, regardless of the rules used to create the bibliographic data.
Learn More in the RDA Section of the NLM Cataloging Page