ICOSL expression has been detected on tumor cells for melanoma, multiple myeloma, colorectal cancer, B‐cell lymphoma, acute myeloid leukemia, glioblastoma multiforme, and rhabdomyosarcoma, whereas it was noted to be present on immune cells infiltrating the tumors for breast, ovarian, gastric, and liver cancers. This evidence concerns the gene ICOSLG and plasma cell myeloma.