GPR133 knockdown reduces cAMP levels in GBM cells in vitro. Forskolin rescues the knockdown phenotype in vitro, in agreement with previous observations.36 The issue of how cAMP signaling regulates tumor growth in GBM has been relatively understudied and equivocal in prior literature.43, 44, 45 Further experiments will be needed to show how GPR133 signaling modulates gene expression, signaling and metabolism. Here, ADGRD1 is linked to glioblastoma.