The prokaryotic mobile genetic element IS903.B is an active variant of IS903. It differs from IS903 and IS102 by 34 and 61 nucleotide substitutions, respectively. The large open reading frame (ORFI) which probably encodes the transposase is conserved in all three IS elements, whereas the smaller open reading frame (ORFII), which codes on the opposite DNA strand and entirely overlaps ORFI, contains an amber stop codon past the middle of ORFII in IS903.B. Experiments using Escherichia coli K12 strains permissive or non-permissive for amber mutations revealed no difference in the cointegration frequency mediated by IS903.B. Therefore, a possible peptide encoded by ORFII on the IS903-related element is unlikely to be necessary for transposition.