RNA-dependent protein kinase (PKR) mediates the antiviral activity of interferon and also has implications in cell growth, differentiation, and apoptosis. On the other hand, the tumor suppressor function of PKR is still controversial. PKR is a serine/threonine kinase that contains two RNA-binding domains (RBD-I and RBD-II) and RBD-I is critical for its activation. Site-directed mutagenesis studies indicated that a single amino acid substitution in RBD-I is sufficient to abolish the interaction of human PKR with RNA. Also, PKR mutants that are unable to bind RNA are inactive in vitro and have no antiproliferative activity in vivo. There have been no reports of mutations in the RNA-binding domains of PKR of tumor cells taken directly from patients. We investigated the presence of mutations in the RBD-I and RBD-II of PKR gene in children with acute lymphoblastic leukemia (ALL). The RNA extracted from bone marrow samples of 15 patients with ALL (5 patients T-lineage; 10 patients B-lineage) was used for to synthesize cDNA and amplify the sequences corresponding to RBD-I and RBD-II. The PCR products were subsequently cloned and sequenced. A point mutation was detected in the RBD-I of PKR from a patient with ALL of T-cell lineage that is located at cDNA nt 50 A --> G (17 Tyr-->Cys). We also found that activation of a PKR mutant by the polyinosinic acid:polycytidylic acid (poly I:C) is impaired when compared with the wild-type PKR. Additional work is required to elucidate whether this point mutation plays a role in the formation and/or maintenance of leukemic cells. To our knowledge, this study is the first example of detection of a mutation in the RBD-I of PKR gene from tumor cells taken directly from patients.