Some GenBank (INSDC) submitters annotate variations on their sequences. You can find such sequence records in the Nucleotide database using this query:
variation[Feature key] AND ddbj_embl_genbank[filter]
If you are focusing on an individual gene, such as human CFTR, you can narrow your search by adding the gene symbol and organism to the search. For example:
CFTR[title] AND Homo sapiens[Organism] AND variation[Feature key] AND ddbj_embl_genbank[filter]
For a human variant of interest, check it against information in the variation databases to see if it is reported by a different submitter elsewhere.
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Keywords: GenBank, variation feature, Nucleotide database search, search strategy