Number of unique k-mers (y axis) by starting position within read (x axis) in an untrimmed E. coli 100-bp Illumina shotgun data set, for k = 17 and k = 32.

The increasing numbers of unique k-mers are a sign of the increasing sequencing error towards the 3′ end of reads. Note that there are only 69 starting positions for 32-mers in a 100 base read.