HHLA2 and neoplasm: Furthermore, primary human RCCs have been found to highly express HHLA2 based on immunohistochemical and RNA-Seq data from The Cancer Genome Atlas (TCGA) database [9, 10, 12, 16], while RCC cell lines rarely express HHLA2 in vitro, indicating a context dependent role of the tumor microenvironment in regulating HHLA2 expression.