# /ensembl/genes/homologs

`POST /api/ensembl/genes/homologs`

Price: 20 credits

List the orthologues and paralogues Ensembl Compara records for a gene

## How to use it

Comparative genomics rather than a name lookup: it answers what the same gene is called in other species. `homology_type` separates orthologues across species from paralogues within one. A gene Ensembl Compara holds but records no homologue of the requested type answers with an empty list, so an empty list is a biological statement rather than a bad identifier; not found means Compara holds nothing under that id at all - a typo, a gene outside the comparative set, or a `species` that does not match the id's own species. Narrow with `target_species` for one species or `target_taxon` for a clade - unfiltered, a well studied vertebrate gene returns well over a hundred homologues. `homology_type` reports the cardinality (ortholog_one2one, ortholog_one2many), which is what tells a one-to-one match from a family expansion.

## Parameters

- `access-token` (string, required)

## Request body

- `timeout` (integer) — Max scrapping execution timeout (in seconds) (default: 300; min: 20; max: 1500)
- `gene` (string, required) — Ensembl stable id of the gene whose homologues are wanted (examples: "ENSG00000139618", "ENSG00000012048"; minLength: 1)
- `species` (string) — Ensembl species the stable id belongs to (default: "homo_sapiens"; examples: "homo_sapiens"; minLength: 1)
- `homology_type` (string) — Which class of homologues to return (default: "all"; one of: "orthologues", "paralogues", "projections", "all")
- `target_species` (string, nullable) — Keep only homologues in this species (examples: "mus_musculus"; minLength: 1)
- `target_taxon` (integer, nullable) — Keep only homologues under this NCBI taxon id (examples: 10090, 40674; min: 1)
- `alignment_sequence` (string) — Return the aligned sequences of each pair in `align_seq`. They dominate the response, so `none` leaves the alignment to `cigar_line` and the identity percentages (default: "none"; one of: "none", "protein", "cdna")
- `count` (integer, required) — Max number of homologues to return (min: 1)

## Response

### 200 — Successful Response

- `@type` (string) (default: "EnsemblHomology")
- `id` (string, required)
- `gene_id` (string, required)
- `target_gene_id` (string) (default: "")
- `homology_type` (string) (default: "")
- `taxonomy_level` (string) (default: "")
- `method_link_type` (string) (default: "")
- `dn_ds` (number, nullable)
- `source` (object, nullable)
  - `@type` (string) (default: "EnsemblHomologyPartner")
  - `id` (string) (default: "")
  - `protein_id` (string) (default: "")
  - `species` (string) (default: "")
  - `taxon_id` (integer, nullable)
  - `identity_percent` (number, nullable)
  - `positive_percent` (number, nullable)
  - `cigar_line` (string) (default: "")
  - `align_seq` (string) (default: "")
- `target` (object, nullable)
  - `@type` (string) (default: "EnsemblHomologyPartner")
  - `id` (string) (default: "")
  - `protein_id` (string) (default: "")
  - `species` (string) (default: "")
  - `taxon_id` (integer, nullable)
  - `identity_percent` (number, nullable)
  - `positive_percent` (number, nullable)
  - `cigar_line` (string) (default: "")
  - `align_seq` (string) (default: "")

## Errors

### 422 — Validation Error

Unknown species

What to do: Check the fields against this schema. A URN with the wrong prefix is the most common cause.

- `detail` (array)
  - `loc` (array, required)
  - `msg` (string, required)
  - `type` (string, required)
  - `input` (any)
  - `ctx` (object)

### 408

The request ran past its time limit

What to do: Raise `timeout` in the request body, up to the maximum this endpoint documents. Lowering `count` or turning off the `with_*` flags also helps, because less work finishes sooner.

### 412

Gene not found

What to do: Retrying will not help: either the entity does not exist, or the input points at a different one.

### 429

Too many requests: a rate limit or a usage window is exhausted

What to do: When the response carries an X-Retry-After header, wait that many seconds and retry: the same number is in the body as `detail.retry_after`, and the limit clears once that window passes. The message in the body names the limit that was hit.

### 500

Something broke on our side

What to do: Retrying will not help. If it keeps happening, send us the X-Request-ID from the response headers.

### 529

Rate limit reached, or the endpoint is overloaded

What to do: Wait at least 30 seconds, then retry.

## Response envelope

Success: Array of objects (may be empty if no results)

Error: Error may coexist with partial results if it occurs mid-execution. Check X-Error header and status code.

Every response carries these headers:

- `X-Error` — Error message text (present only on error)
- `X-Request-ID` — Unique request identifier
- `X-Execution-Time` — Execution time in seconds
- `X-Result-Count` — How many records the body carries. 0 means an empty result, which is a normal answer and not by itself an error. A non-zero count can come back together with X-Error when the failure happened partway through — read this header and X-Error independently.
- `X-Total-Available-Results` — How many records exist for this query, when the endpoint can say. On a `dry_run` request this is the answer and the body is empty. It saturates: the endpoint's documented maximum means 'at least that many', any smaller number is exact.
- `X-Warning` — Present when the request body carried keys this endpoint does not document. They were ignored, so any filter you meant to apply through them did not apply. Check the spelling against this schema and retry.
- `X-Retry-After` — Seconds to wait before retrying. Present only on 429.