Eukaryal tRNA introns are relatively short and range from 12 to 104 nucleotides in e.g., humans [8] . In eukarya, tRNA introns are less abundant than in archaea-e.g., only 6% of human tRNA genes and 20% of yeast tRNA genes are disrupted by introns [8] -and do not display any conserved structural motifs. Rather, the position of the intron is highly invariable in almost all known eukaryotic pre-tRNA genes (Fig. 1B ). An exception to this rule is exemplified by the non-canonical introns found in the circularly permuted tRNA genes of the red alga Cyanidioschyzon merolae (see scheme accompanying Fig. 1B ) [19] .