['The success of bone marrow transplantation in leukemia depends on graft-versus-leukemia (GvL) effects, mediated by cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. These act by CD95L or granule cytotoxins, such as granzyme B (GrB) whose only known inhibitor is proteinase inhibitor 9 (PI-9). Since PI-9 protects cells from CTL, PI-9 may counteract GvL in leukemias. Our aim was to establish methods to analyze the expression and function of PI-9.', 'We screened the most common pediatric ALL subsets, i.e. pre-B, common, and cortical T-ALL, for PI-9 expression, using a reverse transcription polymerase chain reaction (RT-PCR) approach which was correlated to semiquantitative and functional methods established in cell lines and patient probes.', 'In vitro, PI-9 mediated resistance towards CTL and inhibited GrB-induced clevage of caspase 3. In patient-derived ALL cells, PI-9 high and PI-9 low specimens were studied by flow cytometry, RT-PCR, in vitro GrB treatment and NK assay, demonstrating concordant results and PI-9-dependent target cell protection. Analysis of PI-9 in probes from ALL patients showed differential expression, but no correlation with immunotype.'] Is the granzyme B inhibitor PI-9 differentially expressed in all main subtypes of pediatric acute lymphoblastic leukemias?