The correlation of intron positions with module boundaries for 276 ancient, nonrelated genes. Black line: Initial set of 3328 phase-zero introns. Green line: Set 2, comprising 550 phase-zero introns whose positions match in the genes of at least two of five taxa (vertebrates, invertebrates, fungi, plants, and protists). Blue line: Set 3, comprising 118 phase-zero introns whose positions match in at least three taxa. Red line: Set 4, comprising 29 phase-zero introns whose positions match in at least four taxa. (A) The percentage excess of intron positions over the random expectation in module boundaries ([O-E]/E * 100%) as a function of module diameter, where O is the observed number of intron positions in module boundaries, and E the expected number. The horizontal axis gives the module diameter. The range of module diameters from 10–40 Å corresponds approximately to polypeptide chains from 5–45 amino acid residues in length. (B) χ² values for the excess of introns in module boundaries. Thresholds of P = 0.001 (for χ² = 10.8) and P = 0.0001 (for χ² = 15.1) are marked. Only the curves for sets 1 and 4 are given, for clarity; sets 2 and 3 are similar.