Example:
Search: breast cancer
Query Translation: "breast neoplasms"[MeSH Terms] OR ("breast"[All Fields] AND "neoplasms"[All Fields]) OR "breast neoplasms"[All Fields] OR ("breast"[All Fields] AND "cancer"[All Fields]) OR "breast cancer"[All Fields] OR breast cancer[All Fields]

From: NLM Catalog

Cover of The NCBI Handbook
The NCBI Handbook [Internet]. 2nd edition.

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