HHLA2 and neoplasm: Multiple bioinformatic methods were used to investigate the correlation of HHLA2 expression with survival, pathological stage, tumor mutation burden (TMB), microsatellite instability (MSI), tumor microenvironment (TME), immune cell infiltration, and immune checkpoint gene (ICG), and gene functional enrichment was performed by Gene Set Enrichment Analysis (GSEA) and Gene Set Variation Analysis (GSVA).