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In a SAM file generated from Bowtie, I found one entry has '*' in the CIGAR field. What does this mean, bad alignment?
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It means "not available" in the SAM spec. Typically, this happens when the read isn't aligned (RNAME - 3rd field - is also '*' and 0x4 is set in the FLAG field). -
Thanks, arvid.
To what extend, Bowtie sets the FLAG to 0x4 for a read, and make the CIGAR field "not abailable". What are the measure and threshold bowtie uses for setting this?
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Well, using the settings (default or your specific choices), Bowtie found no reportable alignment for that read.
It is not possible to make a general statement about this behaviour here as it depends entirely on the settings you provided bowtie with and the nature of your reads and reference. Check the bowtie manual, typically you should look at alignment and reporting modes - these and their sub-options control alignment sensitivity and reporting filters.Comment
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Hi Arvid,
Thank you very much for the help.Comment
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Love this - good data definitely starts from good input, and poor input can only give relatively poor data. I particularly like the mention of Nanodrop/absorbance based methods for quantification. It's such a toss up if you'll get an accurate reading or what amounts to a randomly generated number, and a lot of library/sequencing related issues can be traced back to poor quant.07-01-2026, 11:43 AM
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I’m not a sequencing expert. I’m a purification scientist who uses NGS to evaluate workflows my group develops. With this perspective, we think about the sample first and the NGS workflow second. The sequencer is an exceptionally honest reporter, but it can only report on what you give it, so whether you get clean, interpretable data from an NGS workflow is largely determined before you begin.
Here are nine questions we think about, in roughly the order they matter, before...
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