why gtf file has 0 length of CDS region

Question

[wangshx]

I retrieve CDS region from gtf file (ensembl v75). When I check the region length of CDS, I am very curious about the following result

library(data.table)

annotation <- fread('Homo_sapiens.GRCh37.75.gtf')

> summary(annotation[annotation$V3=="CDS",]$V5 - annotation[annotation$V3=="CDS",]$V4)    

   Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
    0.0    77.0   115.0   149.2   162.0 21692.0

More than 100 CDS regions have 0 length. Why? Please help me if you know, thanks a lot.

Answer 1

[Devon Ryan]

GTF is 1-based, so those are 1 base long microexons.

[wangshx]

It is new for me that exists 1 base long microexons.