To calculate the exonic mappability, we first construct consecutive reads of
length 90 that are one base pair apart along the exon. The sequences are taken
from the hg19 reference. We then find possible positions across the genome that
the reads can map to allowing for two mismatches. We compute the mean of the
probabilities that the overlapped reads map to the target places where they are
generated and use this as the mappability of the exon.