The translocation t(12;21)(p13;q22) is a frequent nonrandom rearrangem
ent of B-cell lineage childhood acute lymphoblastic leukemia (ALL) whi
ch fuses the TEL and AML1 genes, normally localized to 12p13 and 21q22
, respectively. The crucial chimeric gene, TEL AML1, is transcribed fr
om the der(21) and encodes the 336 NH2 aminoacids of TEL fused to the
majority of the AML1 protein. The t(12;21) is very often associated wi
th loss of the normal, untranslocated TEL allele. These various aspect
s are presented here.