Gene annotation outputs NA values for real genes

Question

[Mozart]

Hello everyone, I have noticed that I was removing with the na.omit() function many known genes that were not successfully annotated with annotationdbi.

res$symbol <- mapIds(org.Mm.eg.db, 
                                   keys=row.names(res),
                                   column="SYMBOL",
                                   keytype="ENSEMBL",
                                   multiVals="first")

do you have any ideas why is this happening, by any chance?

[Kevin Blighe]

Can you provide some example gene IDs? - grazie mille.