*3.2.1 Hyperdiploidy*

Hyperdiploidy is the most prevalent recurrent abnormality in childhood B-ALL. In the World Health Organization classification of tumors of hematopoietic and lymphoid tissues, hyperdiploidy in B-lymphoblastic leukemia (B-ALL), characterized by the presence of 51–65 chromosomes, has been identified as a distinct subtype of B-ALL [49]. In hyperdiploidy, numerical chromosomal gains are nonrandom, with additional copies (usually trisomies) of chromosomes 21, X, 14, and 4 most commonly found in pediatric patients [50]. Despite the presence of nonspecific structural abnormalities, the extra chromosomes are still normal copies of chromosomes. There is a poor understanding of the mechanism involved in inducing hyperdiploidy and its role in leukaemogenesis. Hyperdiploid B-ALL comprises approximately 25–30% of pediatric B-ALL cases [51]; and is often associated with a favorable prognosis with a cure rate greater than 90%, especially when hyperdiploidy is associated with trisomies of chromosomes 4 and 10 [52–54].
