Innate lymphoid cells (ILCs) are tissue-resident lymphocytes that differ from conventional T lymphocytes in having no antigen-specific receptors. ILCs include natural killer (NK) cells, ILC1, ILC2, ILC3 and lymphoid tissue-inducer cell (LTi) subsets. Tumor ILCs are frequently found in various cancers, but their roles in cancer immunity and immunotherapy remain much less clear than those of other lymphocytes, such as T cells and NK cells. The inventors report here the single-cell characterization of blood and gut ILC subsets in healthy conditions and in colorectal cancer (CRC). The healthy gut contains ILC1s, ILC3s, and ILC3/NKs, but no ILC2s. Additional tumor-specific ILC1-like and ILC2 subsets were identified in CRC patients. SLAMF1 (signaling lymphocytic activation molecule family member 1, CD150) was found to be selectively expressed on tumor-specific ILCs (TILCs). More importantly, the inventors show that higher levels of SLAMF1, including protein levels, RNA level as well as levels of SLAMF1+ ILCs were observed in CRC patients. The SLAMF1-high group of rectal cancer patients had a significantly higher survival rate than the SLAMF1-low group, suggesting that SLAMF1 is an anti-tumor biomarker in CRC.