I attempted to retrieve values of dn and ds for human/mouse gene comparisons in Ensembl using BiomaRt (R application), and get the following error (with mart defined as ensembl human reference genome):
t = getBM(attributes = "mmusculus_homolog_dn", filters = "hgnc_symbol", values = "GABRA6", mart = mart) Error in `[.data.frame`(result, , attributes) : undefined columns selected
In contrast, if I try to retrieve some other attribute, such as mouse gene id, it works (e.g.)
> t = getBM(attributes = "mmusculus_homolog_ensembl_gene", filters = "hgnc_symbol", values = "GABRA6", mart = mart) > t  "ENSMUSG00000020428"
So it doesn't seem to be a syntax error. What am I doing wrong in calling the "mmmusculus...dn" attribute (and the same issue with ds)?