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  • Big_Cheese
    Junior Member
    • Jul 2012
    • 7

    #1

    Difference between FPKM and raw reads?

    hey,

    we sent 3 Affected and 3 Control Mice Samples for Sequencing to compare Gene Expression Levels and we got back a bunch of files that were generated with cufflinks. So theoretically we would now compare the Gene expression Levels between Affected and Control to see where they vary and what might cause the phenotype.

    The problem is that I dont know which files to use.
    basically there are 3 excel Files.
    isoform_exp.diff.with.fpkm_denovo
    isoform_exp.diff.with.fpkm_known
    dge_results
    (Differential Gene Expression)

    so the first 2 are the fpkm values(de novo and known) and the last one are basically raw read counts from what I can understand.
    Theroetically it shouldnt matter which file I use because all we need is the ratio from control group to affected to find out which genes are less/more expressed in the affected group.
    But when I try that with the different files I get huge differences that should not be there(if I understand it correctly)
    For example Errc2.
    In dge results it's a 1:1 ratio between control and affected, in fpkm_known it's 1:10 and in fpkm_denovo the gene isnt even listed.

    also I would like to know how they get their FPKM values.
    FPKM means something like Fragments per kilobase of exom per million reads.
    But some Genes have like [S1:40, S2:34, S3:54] in raw reads, but in FPKM it looks like [S1:1.6, S2:0.00005, S3: 1.3]

    I also dont understand how cufflinks calculates p-values.
    There are some genes where FPKM is zero for all the samples, but they still got totally different p values how can that be?
  • IBseq
    Member
    • Jul 2012
    • 56

    #2
    Hi Big Cheese,
    I am new to the sequencing field, but from what you write I might give you some advices:
    first, I understood a lot by reading the manual on cufflinks web site http://cufflinks.cbcb.umd.edu/.

    Have a go and you will see you will find your answers.

    Also there is a recent paper from Trapnell where they did what you are doing basically and you might want to have a go there as well: "Differential gene and transcript expression analysis of RNA-seq experiments with TopHat and Cufflinks"

    Also, as you will read, you might want to run cuffdiff on you samples. Cufflinks it's only assembling your aligned reads, what you want is the difference between you two group samples after assembling them.

    Best,
    ib

    Comment

    • Big_Cheese
      Junior Member
      • Jul 2012
      • 7

      #3
      hm,
      the files I got were already generated with cuffdiff.

      And I read the manual, but it hasn't really helped me
      Last edited by Big_Cheese; 07-30-2012, 11:09 AM.

      Comment

      • DZhang
        Senior Member
        • Jun 2010
        • 177

        #4
        Hi Big_Cheese,

        Call whomever did the sequencing and analysis for you and ask the questions. That's fastest way to have your questions answered.

        Best regards,
        Douglas

        Comment

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