% unmapped reads

[bioinfosm]

Hi all,

For any solexa run, we see 60-80% passing filter reads, of which 70-90% map to the reference sequence. There are quite a few reads that map to adaptors and other random sequences.

Does anyone know of a resource to get an average for those passing-filter reads? that map to adapter, etc and are essentially noise and not useful. I remember it being mentioned in some review as well, but cannot recollect

thanks..

[der_eiskern]

i'm not sure what exactly you want ("an average for those passing-filter reads")

but i've also noticed often two-thirds of the 30-10% of unmappable reads are alignable if you can tolerate a higher error rate.

from cloning random fragments i've noticed that 1/10-1/100 of cloned fragments from a sequencing library will have a truncated adaptor or multiple adaptors. since i didn't get my adaptors from illumina directly this could reflect contamination of my modified IDT oligos. it varied between library preps using the same batch of adaptors.

can you not just make a fasta file of the adaptor and iteratively truncate your read during mapping to such a fast a file to estimate it? i'd be more interested in seeing what what doesn't pass filter looks like, any idea how to get that data from the pipeline?

[Xi Wang]

from cloning random fragments i've noticed that 1/10-1/100 of cloned fragments from a sequencing library will have a truncated adaptor or multiple adaptors. since i didn't get my adaptors from illumina directly this could reflect contamination of my modified IDT oligos. it varied between library preps using the same batch of adaptors.

I am wondering what makes the sequencer not report the adaptor sequences and exactly the very beginning of what we want to sequence. Or it will be quite normal to sequence the tails of adaptors.

can you not just make a fasta file of the adaptor and iteratively truncate your read during mapping to such a fast a file to estimate it? i'd be more interested in seeing what what doesn't pass filter looks like, any idea how to get that data from the pipeline?

Those not passing the filtering usually are with low quality scores, which indicates that the base calling may be incorrect or it is hard to call bases.

[aleferna]

Solexa Sequencing of Paired Ends

Does anybody know what is the expected percent of adapter sequence that one would expect in a Solexa sequencing run?

I'm mapping this solexa paired end run and I had to mask away about 50% of the sample because it maps to the adapters. Is this normal? If so and we do this again, how would you minimize the amount of adapter sequence that you get in the library????

If it is not normal, is there any doc/spec that we can use to complain to the sequencing service and get it done properly?

[Xi Wang]

Does anybody know what is the expected percent of adapter sequence that one would expect in a Solexa sequencing run?

I'm mapping this solexa paired end run and I had to mask away about 50% of the sample because it maps to the adapters. Is this normal? If so and we do this again, how would you minimize the amount of adapter sequence that you get in the library????

If it is not normal, is there any doc/spec that we can use to complain to the sequencing service and get it done properly?

It may depend on what you sequenced. RNA-seq? miRNA-seq?

[aleferna]

just plain human DNA

[Xi Wang]

I have no experience on plain DNA sequencing. But I think your problem may largely due to sample preparation.

[aleferna]

But in what part, is that the main issue is that we provide the samples, but another lab did the Paired End preparation. If there was a problem in the paired end preparation we need to go back to this people and tell them that they made a mistake. Because its such an expensive test this can get really ugly and I don't have any reference to make the point...

[aleferna]

How much do you expect in RNA?