A pair of identical twins develop leukemias, where immature white blood cells proliferate uncontrollably. In each case, the cancer cells analyzed from identical twins displayed an identical chromosomal mutation and this critical mutation was not found in the twins normal cells. The twins developed leukemia at different times in their lives. What is the best explanation for the origin of this chromosomal mutation

The chromosomal rearrangement resulted from a rare somatic mutational event that occurred in utero

Explanation:

Leukemia is a progressive type of cancer where immature white blood cells (leukocytes) proliferate uncontrollably. Next-generation sequencing (NGS) technologies have shown that somatic mutations are relatively common in different types of leukemia such as, for example, acute myeloid leukemia (AML). Moreover, identical twins are developed from one fertilized egg (ovum) that splits and develops into two identical individuals that share all of their genes (i.e., they have exactly the same genetic information). In this case, it is expected that leukemogenesis was initiated in the utero by the occurrence of a chromosomal rearrangement and proceeded early during development as distinct clonalities. In addition, the fact that identical twins developed leukemia at different times in their lives also indicates that this malignancy wasn't caused by only one mutation, but instead it was triggered by multiple mutational events (i.e., leukemia is a multistep cancer caused by secondary genetic changes).