Detailed understanding of GPR81 expression, and in particular its spatial organization in tumors, has been limited by the lack of specific antibodies [36].1 To evaluate GPR81 distribution within tumors, we therefore performed in situ hybridization in tissue microarrays (TMAs) of patient tumor biopsies, having established that the in situ probes were highly specific for GPR81 (Suppl. This evidence concerns the gene HCAR1 and neoplasm.