BCR-mediated tonic signaling is an indispensable requirement for the survival of follicular B (FOB) cells and Burkitt lymphoma (BL) cells. FOB cells of the I-A12% mutant mouse express unfolded protein response and are extremely short lived. Among the myriad molecules activated by unfolded protein response in I-A12% B cells, Xbp1s singularly “hijacked” p110 from p85:p110 heterodimeric PI3K, thereby abating BCR tonic signaling, resulting in their extremely short lifespan. Long-lived normal FOB cells became short lived upon ectopic Xbp1s expression. The proapoptotic Xbp1s role in FOB cells starkly contrasts with its antithetical prosurvival function in plasma cells. Also, tonic signaling and clonal expansion, two important functions mediated by the same BCR, operate in independent and distinct manners. Furthermore, concerning the development of new therapeutic treatment of drug-refractory BL patients, our finding of Xbp1s-mediated rapid death of BL cells brings forth a conceptual advancement based on blocking PI3K heterodimer formation rather than inhibition of PI3K enzyme activity.