Fig 1.

Diagram of three possible transpositions (top) and two possible inversions (bottom) in a circular genome leading to the same gene order. A. Because of the circularity of the genome, there are always three possible transpositions leading to a similar gene order ([B A C] = [A C B] = [C B A]) from a given gene order ([A B C]). Thus, it is not possible to determine which block of genes is concerned by a transposition. B. Similarly, there are always two possible inversions leading to a similar gene order ([-A B] = [A -B]) from a given gene order ([A B]). Thus, it is not possible to determine which block of genes is inverted (A or B). In each example, the transposed or inverted block is underlined. The black arrowheads indicate where the transposed block is inserted. By convention, the circular genomes are read clockwise.