Figure 2. Occurrence frequencies of nucleotides and dinucleotides in introns.

(A) Frequencies of nucleotides and dinucleotides in the first introns and non-first introns. (B) u-values of nucleotides and dinucleotides in the first introns relative to the non-first introns. (C) u-values of nucleotides and dinucleotides in the first introns relative to the non-first introns at different positions. One colored polyline represents a group of u-values of all kinds of nucleotides and dinucleotides for the first introns vs. the non-first introns in one specific position. Since we only focus on whether some nucleotides and dinucleotides are over-represented in the first introns, the differences of nucleotide or dinucleotide content between any two position-specific non-first introns do not be concerned.