Abnormalities of the short arm of chromosome 12 are relatively common in childhood ALL. Approximately 5~7% of children with ALL have cytogenetic evidence of a translocation involving 12p, whereas 3~5% have deletions which suggest the presence of a tumor suppressor gene at this location. Through the use of sensitive molecular techniques, 12p or 12p 12~13 loss of heterozygosity (LOH) has been demonstrated in approximately 25 oyo of childhood ALL cases. FISH mapping has detected a minimum region of overlap for the 12p deletions between the TEL(ETV6) and CDKN1B(KIP1) geness). Recently, chromosomal translocations involving the TEL gene at 12p13 have been cloned in several hematopoietic disorders. In ALL cells with thet(12;21)(p13;q22), the 5' part of TEL is fused with the AML1 gene. Abnormalities of 12p, especially the t(12 ; 21), are more reliably detected by FISH than by classical cytogenetics because the translocated portions 12p and 21q are virtually identical cytologically. When FISH was combined with Southern blotting and RT-PCR, the t(12;21) was identified as a recur-ring chromosomal abnormality in 16~25% of childhood B-lineage ALL. Despite the identification of the fusion partners in the t(12;21), the actual function of the TEL-AML1 and AML1-TEL fusion proteins in promoting malignant transformation is unclear. Whether the TEL-AML1 fusion alone is necessary and sufficient for malignant transformation in ALL and whether TEL inactivation has a role in leukemogenesis is currently unknown. Loss of the CDKN2(p16) gene at 9p21 is a common genetic abnormality in ALL and a variety of other malignancies. Homozygous CDKN2 deketions have been deteced in approximate 15% of B-lineage ALL and 75% of T-lineage ALL cases. The p16 protein functions as an inhibitor of cyclin-dependent kinase 4, and normally acts to stop cell cycle progression. Whether loss of p16 alone is necessary and sufficient for malignant transformation in childhood ALL has not been determined.