PTGDR2 and Carcinoma, Lewis Lung: Although, these in-vitro data suggest anti-tumorigenic properties, the exact role of CRTH2 in cancer is still unclear: CRTH2 expression on circulating CD4 positive cells was elevated in the late stage of non-small cell lung cancer [116], and in an experimental model using Lewis lung carcinoma cells implanted on the back of mice, CRTH2 expression was detected in vascular cells and the growing tumor [117].