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find and fix missed small exons

Project description


An annotation based method to fix small exons missed alignment defects in Nanopore long reads.

The example plot above show the misaligned exon in the middle is find by compaired the reference annotation BED and realigned to the correct position. When we use [minimap2]( to align ONT RNA-seq reads, it is easy to miss the exon with small size, because of the difficulty to find an exact match anchor in these region.


pip install fixalign

Upgrade to a newer version using: pip install fixalign --upgrade

The package is written for python3


fixalign requires four essential arguments.

  • BAM file
  • genome reference fasta (with index fai in the same directory)
  • transcript annotation BED12
  • the target file for the output exon-missed regions information


  • regions which have missed small exons
  • realigned BAM file which are fixed


fixalign [-h] [-s N] [-d float] [-f N] [--ignoreStrand] [--detail]
         [--floatFlankLen] [-o file | --onlyRegion]
         inBam genomeFasta annotBed outRegion

positional arguments:
  inBam                 Input original bam file.
  genomeFasta           Reference genome fasta file, with fai index under the same directory.
  annotBed              Annotated transcripts file in BED12 format.
  outRegion             Output Region file, regions contain missed small exons.

optional arguments:
  -h, --help            show this help message and exit.
  -s, --exonSizeThd N   The threshold of exons size, ignore transcript exons with size > exonSizeThd (default: 100).
                        We use exon size and delta ratio to judge whether the intron region of each read has missed small exons or not. 
                        For more explanation, please go to the NOTES.
  -d, --deltaRatioThd float
                        The threshold of absolute delta ratio, ignore abs(delta ratio) > deltaRatioThd (default: 0.5).
                        Delta ratio = modified margin length / exon size. For more explanation, please go to the NOTES.
  -f, --flankLen N      The extended length on the both sides of realign region (default: 20).
  --ignoreStrand        Consider both strands (default: False).
  --detail              Return all possible missed exons on different transcripts (default: False).
  --floatFlankLen       Flank length can be changed by adjacent indel (default: False).
  -o, --outBam file     Output realigned bam file (default: None).
  --onlyRegion          Only return the Region file without realign process (default: False).


The basic idea for fixalign to check whether the intron regions of reads miss small exons is based on the length compairsion.
  • margin length As the illustrator show above, margin length is defined as the extra length of read exons compaired to the annotation exons (e.g. a, b and m1+m2 is margin length correspond to the annotated two transcripts). We find the reads introns if the intron overlaps with the annotated exon, and then check whether the margin length close to the exon length. It is easy to think that margin length should be close to the exon length if they come from the annotated exon. But as we find out that these false aligned region (realign region) usually contain many of indels (insertion and deletion) and the margin length is smaller than exon length in the most case.
  • flank length In order to define the range of the realign region. we set the region start = min(annoted splice site, read splice site) - flank length and the region end = max(annoted splice site, read splice site) + flank length
  • modified margin length we count the number of indels in the realign region and define modified margin length (modified margin length = margin length + insertions - deletions) instead of margin length to compair with the exon length.
  • delta ratio delta ratio = (modified margin length - exon length) / exon length. We choose exon length <= exonSizeThd and delta ratio <= deltaRatioThd to filter out the false positives and record the left regions as exon-missed region. The regions information is output in outRegion.
  • --detail One Read intron may be overlapped with exons on multiple transcripts. As default, we only return the annotated transcript with minimum delta raio as the correct reference. You can set --detail to keep all possible transcripts reference. In the realign process, we only keep one realign result among different transcript, the one with highest realignment score will be kept.

We use parasail to do the global pairwise alignment between read sequence and annotated exon sequence in the realign region. We only keep the realign result if realigned score > original score. ATTENTION: the original score does not refer to the AS field in BAM if provided. We calculate the realigned score and origial score based on the realignment and original alignment in the realign region, and the score is equal to the matched bases count - edit distance. As different alignment tools may have different score system, we do not change the AS of NM field in BAM if provided.

  • --onlyRegion Although we default to return the realigned result in -o, --outBam file, you can set the --onlyRegion to skip the realign process (although the realign process is not the bottleneck at present).
  • --ignoreStrand This argument is used if the original reads is not stranded RNA-seq. We will try both strands to find the overlapped exons.


git clone
cd examples
fixalign ex.bam ex.fa ex_annotation.bed ex_realign_region -f 20 --ignoreStrand -o realn.bam


  • Zhen Liu for building and maintance
  • Wu Wei and Chenchen Zhu for advising


Welcome for all suggestions, bug reports, feature request and contributions. You can leave an issue or open a pull request.


If you use this tool, please consider citing our publication

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