Update README.md

README.md

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 [<img src="https://github.com/bzhanglab/SEPEPquant/blob/main/doc/protein-and-peptide-distribution.jpg" width=400 class="center">](https://github.com/bzhanglab/SEPEPquant)
 
+Among the 19449 protein coding genes annotated in a RefSeq database, 14698 (75.6%) have more than one protein isoforms, and 3409 (17.5%) have 10 or more protein isoforms (Fig. 1a). Most of isoforms from the same gene have very high sequence similarity (>90%, Fig. 1b). However, among the 11809 genes with three or more protein isoforms, 6165 (52.2%) have at least one pair of isoforms with a sequence similarity lower than 90%, or an average of one amino acid difference in every 10 amino acids, suggesting the possibility to identify isoform-discriminating peptide sequences for a substantial number of genes.
+To further assess the challenge and opportunities of isoform characterization using shotgun proteomics, we performed in silico trypsin digestion of the RefSeq protein database to generate fully tryptic peptides with length 7 to 50 and no missed cleavage. Among the 1,883,206 resulting peptide sequences, 2.8% could be associated to multiple genes (i.e., multi-genes peptides), 13.6% to genes with a single protein isoform (i.e., single isoform peptides), and 83.5% to genes with more than one isoforms (i.e., multi-isoforms peptides). Within the group of multi-isoforms peptides, around half could be mapped to all protein isoforms of a gene and thus providing no information for isoform discrimination (i.e., non-discriminative peptides); however, another half, or 246,615 peptides, could be uniquely mapped to one isoform (i.e., fully discriminative peptides) or a subset of isoforms (i.e., partially discriminative peptides) (Fig. 1c).
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 [<img src="https://github.com/bzhanglab/SEPEPquant/blob/main/doc/parsimony-selection.jpg" width=400 class="center">](https://github.com/bzhanglab/SEPEPquant)
 
 [<img src="https://github.com/bzhanglab/SEPEPquant/blob/main/doc/sepep-quantification.jpg" width=400 class="center">](https://github.com/bzhanglab/SEPEPquant)