One-step and 2-step mechanisms of variant Ph. A variant Philadelphia chromosome (Ph) is generated from translocation of 1 or more partner chromosomes in addition to chromosomes 9 and 22. One-step mechanism of variant Ph: 3 breaks and reciprocal rejoining occur at the same time on chromosome 9, 22, and partner chromosome. Two-step mechanism of variant Ph: the standard t(9;22) is formed and then the segment of der9 exchanges material with the variable partner chromosome. V indicates variable partner chromosome; G, green signal; R, red signal; and F, fusion signal.