Sphingosine 1-phosphate receptor 4 (S1PR4) expression in melanoma and its effect on survival and prognosis. (A) Comparison of S1PR4 expression in 33 tumors and their adjacent tissues. Relationship between high and low expression of S1PR4 and survival (B) and progression-free survival (C) in patients with skin cutaneous melanoma (SKCM). Statistical significance was calculated using Wilcoxon test and annotated as *p < 0.05, **p < 0.01, ***p < 0.001. Red, tumor; blue, normal; purple, metastasis of SKCM.