I'm trying to get some `entrez` ids from some gene names using [biomaRt][1]

In my example below I have 2 genes `Igha` and `Mlc1` for mus musculus

My version of biomaRt is `biomaRt_2.38.0`

    library("biomaRt")
    mart <- useMart(biomart = "ensembl", dataset = "mmusculus_gene_ensembl")
    getBM(attributes=c("ensembl_gene_id","mgi_symbol","entrezgene"), values = c("Igha", "Mlc1"), bmHeader = T, filters = "mgi_symbol", mart = mart)
    #      Gene stable ID MGI symbol NCBI gene ID
    #1 ENSMUSG00000095079       Igha           NA
    #2 ENSMUSG00000035805       Mlc1       170790

I know that some Ensembl gene ID do not have Entrez gene ID https://www.biostars.org/p/275619/

But in this case, I'm able to find the Entrez gene ID from Ensembl

http://www.ensembl.org/Mus_musculus/Gene/Summary?db=core;g=ENSMUSG00000095079;r=12:113254830-113260236

Which is `Refseq Gene ID 238447`

Maybe https://www.biostars.org/u/7019/ will have an answer :)

  [1]: https://bioconductor.org/packages/release/bioc/html/biomaRt.html
The gene listed on the gene page is not actually a proper external reference and doesn't come through BioMart. That comes through a different pipeline and it's not reliable and we're getting rid of it.