Structure of Tc1 / mariner transposons. The transposons can be grouped by their different TIR structure. IR/DR TEs have long TIRs with two transposase binding sites (TBSs) per TIR in the same orientation (direct repeats). Simple-structured TEs have short TIRs with only one TBS. Frog Prince has 26 bp-long DRs, but the TBSs are only 21 bp. DR: direct repeat; ssTIR: simple-structured TIR; TBS: transposase binding site; TE: transposable element; TIR: terminal inverted repeat; IR/DR: i nverted r epeat/d irect r epeat.