What does "-" allele mean in dbSNP?

[PeteH]

What does the "-" observed allele mean in dbSNP? I am using dbSNP128 for the mouse mm9 reference genome and there are several thousand such SNPs, for example rs8253350 (http://genome.ucsc.edu/cgi-bin/hgc?h...28&i=rs8253350). I have been unable to find an explanation on the UCSC or dbSNP websites.
Many thanks.

[gringer]

Well, the class for the "SNP" you pointed at was insertion, so I'm guessing that the '-' means a deletion (or a non-insertion). It's somewhat common notation for skipped bases in alignment files.

[PeteH]

Thanks for your reply, gringer. I thought it likely meant a deletion but I was struggling to find clear documentation that said so. The SNP I linked to is listed as:
Observed: -/T
Reference allele: -
To clarify, does this mean the reference sequence contains a deletion at this site and that the T allele was been observed in other samples? I find it rather confusing dealing with INDELs in the reference; to me it is much clearer to visualise INDELs in a sample compared against the reference.

[gringer]

To clarify, does this mean the reference sequence contains a deletion at this site and that the T allele has been observed in other samples?

Yes, that would be my interpretation of those statistics. Other dissenting (or corroborating) views are welcome, though.