Ovarian endometriosis lesions also expressed all histamine receptors, with significantly increased levels of HRH1 (1.85, 0.70–5.45 vs. 0.11, 0.04–0.36; p < 0.0001), HRH2 (1.40, 0.39–3.57 vs. 0.14, 0.01–0.19; p < 0.0001), HRH3 (1.97, 0.79–6.91 vs. 0.54, 0.12–1.30; p = 0.0008), and HRH4 (7.59, 2.91–13.11 vs. 0.11, 0.03–0.96; p < 0.0001) compared to their respective controls (Figure 4I–L). Here, HRH1 is linked to ovarian endometriosis.