[BioC] understanding multiples matches between probesets and entrezgene (biomart)

[Juliet Hannah]

All,

I understand the concept of multiple probesets corresponding to one
identifier. But what is the meaning of
a probeset corresponding to multiple identifiers?  And below, given
that 220547_s_at has a match,
why should another row be returned with NA.

Did I happen to choose a few probesets where the gene definition is
changing, or am I misunderstanding
something else, such as the biomart syntax.

Thanks,

Juliet

library("biomaRt")
probeSets <- c("219666_at", "220547_s_at", "218034_at")
ensembl = useMart("ensembl")
ensembl = useDataset("hsapiens_gene_ensembl",mart=ensembl)

getBM(attributes = c("affy_hg_u133a", "entrezgene"), filters =
"affy_hg_u133a",values = probeSets, mart = ensembl)


  affy_hg_u133a entrezgene
1   220547_s_at      54537
2     218034_at      51024
3   220547_s_at         NA
4     219666_at      64231
5   220547_s_at     414241
6   220547_s_at     439965




> sessionInfo()
R version 2.15.0 (2012-03-30)
Platform: x86_64-unknown-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
 [7] LC_PAPER=C                 LC_NAME=C
 [9] LC_ADDRESS=C               LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] biomaRt_2.12.0      BiocInstaller_1.4.6

loaded via a namespace (and not attached):
[1] RCurl_1.91-1 tools_2.15.0 XML_3.9-4