Normalized fragile amino acid usage (NFAU) (controlling for GC-content) of proteins encoded by multi-exon genes and single-exon genes in the human, mouse, and fly genomes.

NFAU, median (first and third quartiles) of normalized fragile amino acid content of encoded proteins; P-value, significance of the comparison with multi-exon genes determined by two-sided Wilcoxon-rank sum test that tests for a linear shift in distribution locations; Ratio, median NFAU relative to the median NFAU for multi-exon genes.