S1PR1 and breast cancer: Moreover, the S1P1 expression level and Foxp3+ Treg density were substantially increased in BC tissues compared with tumor-adjacent tissues (Fig. 1c, d, P < 0.05), and interestingly, tumor S1P1 expression was significantly correlated with the number of Foxp3+ Tregs in tumor specimens from 116 BC patients (Fig. 1e, P = 0.035, R = 0.196).