Presence of premature termination codons in spliceosomal introns, as a function of intron size (3n, 3n + 1, 3n + 2) and insertion frame (frame 0, 1 and 2) within the coding sequence. (a) A PTC is generated for all retained introns inserted in frame 2 and containing GTGAGT or GTAAGT as the 5'ss sequence, whatever their length; 209 introns are concerned, that is, 19.3% of all intron-containing genes. (b) PTCs (TAA) are also detected in the BP of 3n + 2 introns in frame 0, 3n + 1 introns in frame 1 or 3n introns in frame 2 if the S2 distance is indeed 1 bp. (c) The main 3'ss is CAG, but, in about 10.5% of the introns, TAG is also used. This sequence generates a PTC for 3n introns inserted in frame 0, 3n + 2 introns in frame 1 and 3n + 1 introns in frame 2. Overall, conserved intron motifs are present in about 50% of the PTC-containing introns.