Hi All,
I have a question regarding trimming of adapters in NGS data. I have previously analyzed RNAseq data using brtim (https://www.sciencedirect.com/science/article/pii/S0888754311001339) without using adapter sequence. I am now analyzing smallRNAseq data, and I am using adapter sequence as adapter.fa using BBduk.sh from BBmap tools. Could you please clarify under what circumstances one would need to provide/know adapter sequence and when it is not necessary? 
This is a good question for the beginners who are starting with the NGS analysis.

> In common short read sequencing, the DNA insert (original molecule to
> be sequenced) is downstream from the read primer, meaning that the 5'
> adapters will not appear in the sequenced read. But, if the fragment
> is shorter than the number of bases sequenced, one will sequence into
> the 3' adapter. To make it clear: In Illumina sequencing, adapter
> sequences will only occur at the 3' end of the read and only if the
> DNA insert is shorter than the number of sequencing cycles (see
> picture below)!

This is true in case of smallrna sequencing like microRNAs and piRNAs which are between 16-33 nt long. That means even with 50bp single end sequencing, 3' adapter sequences will occur. This is generally not the case with RNAseq where the gene/transcript fragments are way longer than the sequencing length and adapter sequences do not accur.

![enter image description here][1]

For details: https://www.ecseq.com/support/ngs/trimming-adapter-sequences-is-it-necessary

Also, if you are not sure which adapters are used for the sequencing, you can use an adapter predicting software like DNApi that will predict the most common adapter sequence which can be used by the tools like cutadapt or STAR before the mapping.
![enter image description here][2]
Paper: Tsuji, J., & Weng, Z. (2016). [DNApi: a de novo adapter prediction algorithm for small RNA sequencing data][3]. PloS one, 11(10), e0164228.

link: https://github.com/jnktsj/DNApi

I hope this helps. 


  [1]: https://www.ecseq.com/support/ngs/img/fragmentsize.png
  [2]: http://journals.plos.org/plosone/article/figure/image?size=large&id=10.1371/journal.pone.0164228.g001
  [3]: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0164228