VTCN1 and nonpapillary renal cell carcinoma: We found that the mRNA expression level of HHLA2 was positively and significantly correlated with PD-L1 (Fig. 1b, P < 0.05), PD-L2 (Fig. 1c, P < 0.001) and B7-H6 (Fig. 1f, P < 0.0001), while it was negatively and significantly correlated to B7-H3 (Fig. 1d, P < 0.05), and there was no association between HHLA2 and B7-H4 (Fig. 1e, P > 0.05), suggesting a certain similarity of the potential biological function of HHLA2 with other immune checkpoint molecules in the tumor microenvironment of human ccRCC.