Age of adjacent pairs in different groups. The adjacent alternative pairs are assigned one of the three labels: "young", "middle" and "old", based on the multiple genome alignment (See text and method). The figures show the distributions of age in different groups. Most LNK pairs are old: the younger the age, the fewer pairs. IND shows the opposite trend: The older, the fewer. ME group is a mixture of LNK and IND. The numbers of young and old pairs in ME are roughly equal. These distributions indicate possible different evolutionary courses of the three groups. All the distributions are consistent in human and mouse. However, the label assignment procedures are different in human and mouse, so the amount of the data of each label can not be compared directly.