Skillintermediate

NCBI Protein API Reference

**Example -- search human insulin:** ``` GET esearch.fcgi?db=protein&term=insulin+AND+homo+sapiens[Organism]&retmax=5&retmode=json ``` Response (JSON): ```json { "esearchresult": { "count": "1523", "retmax": "5", "idlist": ["116734704", "AAA59172.1", "NP_000198.1", ...], "querytranslation": "insulin AND \\"Homo sapiens\\"[Organism]" } } ```

Claude Code Knowledge Pack7/10/2026

Overview

NCBI Protein API Reference

Overview

Protein sequence records (RefSeq, GenBank, UniProt imports) accessible via NCBI E-utilities with db=protein.

Base URL

https://eutils.ncbi.nlm.nih.gov/entrez/eutils/

Authentication

API key (recommended): Register at https://www.ncbi.nlm.nih.gov/account/ and append &api_key=YOUR_KEY.
Without key: 3 requests/second. With key: 10 requests/second.
Provide tool and email parameters for identification.

Key Endpoints

1. ESearch -- Search protein records

GET esearch.fcgi?db=protein&term=QUERY&retmax=N&retmode=json

Param	Description
`term`	Search query (Entrez syntax). Fields: `[Protein Name]`, `[Organism]`, `[Accession]`, `[Gene Name]`
`retmax`	Max IDs returned (default 20, max 100000)
`retstart`	Offset for pagination
`usehistory`	`y` to store results on server (use with large sets)

Example -- search human insulin:

GET esearch.fcgi?db=protein&term=insulin+AND+homo+sapiens[Organism]&retmax=5&retmode=json

Response (JSON):

{
  "esearchresult": {
    "count": "1523",
    "retmax": "5",
    "idlist": ["116734704", "AAA59172.1", "NP_000198.1", ...],
    "querytranslation": "insulin AND \\"Homo sapiens\\"[Organism]"
  }
}

2. EFetch -- Retrieve protein records

GET efetch.fcgi?db=protein&id=IDS&rettype=TYPE&retmode=MODE

rettype	retmode	Output
`fasta`	`text`	FASTA sequence
`gp`	`text`	GenPept flat file
`gp`	`xml`	GenPept XML (INSDSeq)
`acc`	`text`	Accession list
`seqid`	`text`	SeqID list
`ft`	`text`	Feature table

Example -- fetch FASTA for NP_000198.1 (human insulin):

GET efetch.fcgi?db=protein&id=NP_000198.1&rettype=fasta&retmode=text

Response:

>NP_000198.1 insulin preproprotein [Homo sapiens]
MALWMRLLPLLALLALWGPDPAAAFVNQHLCGSHLVEALYLVCGERGFFYTPKTRREAED
LQVGQVELGGGPGAGSLQPLALEGSLQKRGIVEQCCTSICSLYQLENYCN

Example -- fetch GenPept XML for multiple IDs:

GET efetch.fcgi?db=protein&id=NP_000198.1,NP_001278826.1&rettype=gp&retmode=xml

3. ESummary -- Brief record summaries

GET esummary.fcgi?db=protein&id=IDS&retmode=json

Returns: accession, title, organism, length, taxonomy, create/update dates.

4. ELink -- Find related records

GET elink.fcgi?dbfrom=protein&db=gene&id=NP_000198.1

Links protein to gene, nucleotide, structure, taxonomy, etc.

Common Search Patterns

# By accession
term=NP_000198.1[Accession]

# By gene name + organism
term=BRCA1[Gene Name] AND human[Organism]

# RefSeq only
term=insulin AND srcdb_refseq[Properties]

# By sequence length range
term=100:500[Sequence Length] AND kinase[Protein Name]

Rate Limits

Without API key: 3 requests/second
With API key: 10 requests/second
Large batch downloads: use usehistory=y with WebEnv/query_key, then fetch in chunks of 500