Good Morning Andrea:

the chrUn is unplaced sequence, chromosome unknown:

http://www.hgsc.bcm.tmc.edu/ftp-archive/Btaurus/fasta/Btau20070913-freeze/README.Btau20070913.txt

The Ensembl names for these are simply Un without the chr prefix.

Beware of cow sequence versions, there are a number of cow genome releases
from different assemblies:

mysql --user=genome --host=genome-mysql.cse.ucsc.edu -A \
     -e 'select * from dbDb where name like "bos%";' hgcentral


--Hiram

Andrea Edwards wrote:
> Hello all
> 
> Please can you advise me what sequences the ChrUn numbers returned by 
> liftover refer to. For example a liftover performed on bos taurus from 
> MD3 to Btau4 returned mappings such as
> 
> chrUn.004.1000    244    245
> 
> A search in entrez for a sequence  'chrUn.004.1000' or '004.1000' 
> returns nothing.
> 
> I do not know whether these sequences refer to unlocalized sequences or 
> unplaced sequences (see definitions below)
> 
> thanks
> 
> Unlocalized sequence:
>     A sequence found in an assembly that is associated with a specific
>     chromosome but cannot be ordered or oriented on that chromosome.
> 
> Unplaced sequence:
>     A sequence found in an assembly that is not associated with any
>     chromosome.
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