I want to retrieve the canonical sequence for some proteins from ensembl, based upon their UniProt IDs.

```r
library(biomaRt)

ensemblMart <- useMart(
  biomart = "ensembl",
  dataset = "hsapiens_gene_ensembl"
)

(result <- getBM(
  attributes = c("uniprot_swissprot", "peptide"),
  filters    = "uniprot_swissprot",
  values     = "P31749",
  mart       = ensemblMart,
  bmHeader   = TRUE
))
```

This particular protein has [two isoforms][1], both of which are returned. I only care about the canonical form, so how can I distinguish the isotopes?

I suspect that I need to return another of the attributes from the mart

```r
listAttributes(ensemblMart)
```

but I don't know which one.

 [1]: http://www.uniprot.org/uniprot/P31749#sequences
<p>There is no canonical defined in BioMart.</p>
