Because it is a cryptic translocation which usually escapes diagnosis on conventional cytogenetic (CC) study due to the similarity of the changed bands, molecular cytogenetic tools such as fluorescence in situ hybridization (FISH) are needed to determine the incidence of the ETV6-RUNX1 fusion in the Tunisian B-lineage ALL pediatric cases that had normal karyotypes or random chromosome aberrations at diagnosis and to compare our findings to those previously described in literature. The gene discussed is RUNX1; the disease is acute lymphoblastic leukemia.