Cluster and PC analysis of SCUB frequencies based on intron number and exon position in algal and land plant genomes. (A, B) Cluster analysis, (C, D) Principal component analysis. PC1, PC2: coefficients associated with the first two extracted principal components. A, C: SC frequency in genes comprising from zero to nine introns. B, D: SC frequency in exons of gene sequences arranged into between two and ten exons.