Reported cases with a structurally abnormal X chromosome were compiled. These included 17 balanced and 26 unbalanced X-autosome translocations, each with inactivation of either a derivative X or a derivative of any of the autosomes. A further 52 cases with various structural rearrangements were studied. The shortest late-replicating segment in each arm pter leads to p21 and q13 leads to qter. In both cases, they were detected in all or most metaphases, thus making the results convincing. In one case, the distal part of Xq, q25 or 26 leads to qter was probably inactivated in a small proportion of the cells. It appears reasonable to assume that the former two segments and probably also the third include an "inactivation center(s)." In a male with a 46,Y,dup(X)(q13q22), no part of dup X replicated late although it contained extra chromosome material.