products, including PGD2 itself, were detected at levels too low for accurate quantification and showed no significant differences. Overall these data suggest that L-PGDS may act as a lipocalin rather than as a prostaglandin synthase [15,35,36]. What lipid- signalling molecule L-PGDS is bound to, or even if L-PGDS itself acts directly as a signalling molecule binding to an as yet unidentified surface receptor, remains to be determined. The ability of L-PGDS to act as a circulating factor could also be responsible for the relatively strong phenotype observed in liver, despite the down regulation in L-PGDS expression. Overall, we conclude that L-PGDS compensates in part for a loss of PPARc2 in vivo. Undoubtedly the phenotype is complex, containing aspects of both L-PGDS and PPARc2 ablations alone, as well as additive and synergistic effects of the loss of both molecules. 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