Masked Philadelphia chromosome caused by translocation (9;11;22).

In a patient with chronic myelocytic leukemia (CML), chromosome analysis revealed a translocation involving chromosomes No. 9, 11, and 22, with three break points, thus giving origin to a so-called "masked" Philadelphia chromosome (Ph1). A review of similar cases reported in the literature indicates that a masked Ph1 is very rare, that the chromosomes involved vary from case to case, and that in most cases the pattern of the rearrangement is quite different from that of two- and three-chromosome variant Ph1 translocations.