Bio Ensembl REST

Version Compatibility

Reference examples tested with: requests 2.31+, Ensembl REST API (release 110+); Ensembl release schedule is roughly quarterly

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show requests
• API surface: check release notes at https://rest.ensembl.org

Each Ensembl release has an archive REST endpoint (e.g. https://e110.rest.ensembl.org) for reproducibility.

Ensembl REST

"Pull Ensembl-native gene / transcript / variant data programmatically" -> Ensembl REST is distinct from NCBI Entrez and BioMart. It is the right answer for: stable Ensembl IDs, transcript / exon structure, VEP (Variant Effect Predictor) annotation, Compara orthologs at vertebrate scale, regulatory feature annotation, and any workflow rooted in Ensembl's coordinate system.

Two facts dominate Ensembl REST work: (1) the 15 req/sec / 55,000 req/hour rate limit — high enough for hundreds of queries, low enough that bulk work (>5,000) belongs in BioMart instead; (2) versioned archive endpoints — https://e110.rest.ensembl.org pins to release 110 for reproducibility, while https://rest.ensembl.org follows the current release.

• Python: requests.get('https://rest.ensembl.org/...')
• Web: https://rest.ensembl.org (interactive doc with try-it-now)
• R: biomaRt for bulk (see biomart-queries); REST via httr

Required Setup

import requests
import time

BASE = 'https://rest.ensembl.org'
HEADERS = {'Accept': 'application/json'}
SLEEP = 0.07   # 15 req/sec ceiling

No API key required. Respect Retry-After header on 429.

Ensembl divisions

Ensembl is divided by clade. Different REST hosts:

For non-vertebrate work, check ensemblgenomes.org mirrors. As of 2024, Ensembl Genomes was being consolidated; check current host.

Version pinning

For any published analysis, pin the release. Ensembl releases change gene model versions, exon coordinates, and transcript annotations — re-running a pipeline a year later against the live endpoint may produce different results.

Major endpoint groups

Full reference: https://rest.ensembl.org (interactive).

The symbol-vs-ID stability problem

Gene symbols are unstable (MARCH1 -> MARCHF1 in 2020 due to Excel autocorrect; SEPT* family also renamed). Ensembl Gene IDs (ENSG...) are stable across releases when the gene model is preserved.

Best practice:

1. Resolve symbol -> Ensembl ID once at pipeline start: /lookup/symbol/{species}/{symbol}.
2. Persist the Ensembl ID.
3. Run downstream queries by ID, not symbol.

Symbol-based endpoints are convenient for interactive use; ID-based endpoints are for reproducible pipelines.

Rate-limit math

Respect Retry-After header on HTTP 429. For >5,000 queries, switch to BioMart bulk export (see biomart-queries) — BioMart has separate, more permissive limits.

VEP (Variant Effect Predictor)

VEP via REST is the right call for ad hoc variant annotation. For batch variant annotation (>1000 variants), download VEP and run locally (variant-calling/variant-annotation skill).

REST modes:

• /vep/{species}/region/{region}/{allele} — single variant by coordinate
• /vep/{species}/id/{variant_id} — by dbSNP / Ensembl variant ID
• /vep/{species}/hgvs/{hgvs_notation} — by HGVS notation

VEP returns rich annotation: consequence (missense, synonymous, intron), SIFT/PolyPhen scores, gnomAD frequencies (if available), ClinVar significance.

Compara homology

Compara orthology calls via Ensembl REST are covered in detail in ortholog-inference (database-access view). The relevant endpoint:

• /homology/symbol/{species}/{symbol} — all orthologs across Ensembl species
• /homology/id/{ensembl_id} — same, by ID
• ?target_species= to restrict to one target
• ?type=orthologues or paralogues to filter

Code patterns

Symbol -> stable Ensembl Gene ID

Goal: Resolve a gene symbol to its current Ensembl Gene ID once, then use the ID for all downstream queries.

Approach: /lookup/symbol/{species}/{symbol} returns the canonical record.

Reference (requests 2.31+):

import requests
import time

BASE = 'https://rest.ensembl.org'
HEADERS = {'Accept': 'application/json'}


def get_with_retry(url, params=None, max_retries=3):
    for attempt in range(max_retries):
        r = requests.get(url, params=params, headers=HEADERS)
        if r.status_code == 429:
            time.sleep(int(r.headers.get('Retry-After', '5')))
            continue
        r.raise_for_status()
        return r
    raise RuntimeError(f'Failed after {max_retries} retries')


def symbol_to_ensembl(species, symbol):
    r = get_with_retry(f'{BASE}/lookup/symbol/{species}/{symbol}')
    return r.json()


info = symbol_to_ensembl('human', 'BRCA1')
print(f'  Ensembl Gene ID: {info["id"]}')
print(f'  Biotype:         {info["biotype"]}')
print(f'  Chromosome:      {info["seq_region_name"]}:{info["start"]}-{info["end"]}')
print(f'  Strand:          {info["strand"]}')

Sequence retrieval by Ensembl ID

def get_sequence(ensembl_id, seq_type='cdna'):
    '''seq_type: cdna, cds, protein, genomic'''
    r = get_with_retry(f'{BASE}/sequence/id/{ensembl_id}',
                       params={'type': seq_type, 'content-type': 'application/json'})
    return r.json()


prot = get_sequence('ENSG00000139618', seq_type='protein')
print(f'  Length: {len(prot["seq"])} aa')

Overlap: what genes are in this region

def genes_in_region(species, region):
    '''region as "chr:start-end" e.g. "17:43000000-44000000".'''
    r = get_with_retry(f'{BASE}/overlap/region/{species}/{region}',
                       params={'feature': 'gene'})
    return r.json()


for g in genes_in_region('human', '17:43000000-43200000'):
    print(f'  {g["external_name"]:<12} {g["id"]} {g["biotype"]:<20} {g["start"]}-{g["end"]}')

VEP for a single variant

Goal: Get full annotation for a variant by coordinate.

Approach: /vep/{species}/region/{region}/{allele} returns transcript consequences, SIFT/PolyPhen, frequencies.

Reference (Ensembl REST release 110+):

def vep_region(species, region, allele):
    r = get_with_retry(f'{BASE}/vep/{species}/region/{region}/{allele}')
    return r.json()


# BRCA1 missense variant in GRCh38 coordinates (rest.ensembl.org defaults to GRCh38);
# for GRCh37 coords use https://grch37.rest.ensembl.org instead.
results = vep_region('human', '17:43044295-43044295:1', 'A')
if results:
    for tc in results[0].get('transcript_consequences', [])[:5]:
        print(f'  {tc["gene_symbol"]:<8} {tc["consequence_terms"]}')
        if 'sift_prediction' in tc:
            print(f'    SIFT: {tc["sift_prediction"]} ({tc.get("sift_score", "?")})')

Compara orthologs (Compara via REST)

def orthologs(species, symbol, target_species=None):
    params = {'type': 'orthologues'}
    if target_species:
        params['target_species'] = target_species
    r = get_with_retry(f'{BASE}/homology/symbol/{species}/{symbol}', params=params)
    return r.json()['data'][0]['homologies']


for o in orthologs('human', 'BRCA1', target_species='mouse'):
    print(f'  {o["target"]["species"]:<15} {o["target"]["id"]}  type={o["type"]}  confidence={o.get("confidence")}')

Batch lookup with rate-limit handling

def batch_symbols(species, symbols):
    out = {}
    for sym in symbols:
        try:
            out[sym] = symbol_to_ensembl(species, sym)
        except requests.HTTPError as e:
            out[sym] = {'error': str(e)}
        time.sleep(0.07)  # 15 req/sec ceiling
    return out

Archive endpoint for reproducibility

# Pin to release 110
ARCHIVE = 'https://e110.rest.ensembl.org'
r = requests.get(f'{ARCHIVE}/lookup/symbol/human/BRCA1', headers={'Accept': 'application/json'})
print(r.json()['id'])
# Re-runs against e110 in 2030 will return the same Gene ID even if the live release has moved on.

LD between two variants

def ld_pairwise(species, var1, var2, population='1000GENOMES:phase_3:CEU'):
    r = get_with_retry(f'{BASE}/ld/{species}/pairwise/{var1}/{var2}',
                       params={'population_name': population})
    return r.json()

Failure modes