To verify that the selective antiproliferative effect of CHIR99021 in FHIT-deficient lung cancer cells was due to the synthetic lethal interaction between FHIT and GSK3β rather than the compound’s off-target effect, we silenced FHIT or GSK3β with specific siRNAs in lung cancer cells and measured cell viability. The gene discussed is FHIT; the disease is lung carcinoma.