GENSCAN detecting the missing exons. (A) After aligning expressed sequence tags (ESTs) to the genomic sequence, the promoter of the gene may be far upstream. A search for the promoter in such large regions is an error-prone process. (solid-line boxes) Exons identified by aligning ESTs to genomic sequence. (arrow) The true transcription start site (TSS). (B)GENSCAN is used to find the missing exons of the gene (dotted-line boxes). Even the external exon (thick-line box) of the gene might not be found by GENSCAN, but we successfully located the promoter to a 2.5-kb region. Finding the promoter in this 2.5-kb region is a much easier job.