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Old 10-03-2013, 02:06 AM   #1
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Default MACS and SAM flags for paired end


I am analysing paired end sequences from a chipseq. I align with bowtie2 and feed the sam output to Macs.
What are the criteria by which macs excludes/includes alignments?
For instance, as far as I understand, flags for valid alignments are 83 and 99. How does macs consider alignments with "non valid" flags such as 81 and 97?
I could not find a clear answer in the manual.

Thank you for the help,

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Old 10-03-2013, 07:47 AM   #2
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Following my question:
I have run some tests and it looks like peaks are generated only from reads with the "right" flags (99, 147, 83, 163) and the rest is ignored.
However, I would appreciate if someone could confirm this.

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Old 10-03-2013, 11:40 AM   #3
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Yes, Tao has already confirmed this:!se...M/KKVWRXl24CYJ

----By Tao:
Both MACS14 and MACS2 favor SAM or BAM format for your Pairend data.

MACS1.4: Pair-end data will be read and simply filtered using flags in SAM file, only 5' ends will be kept.

MACS2: Option 1) Similar to MACS1.4; Option 2) MACS2 will bypass prediction for fragment size, and use real fragments during reads mapping to pile up signals ( done by Ben Schiller). The later option can only be found in MACS2 > version To turn it on, use --format BAMPE.


Last edited by harryzs; 10-03-2013 at 11:43 AM.
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