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Protocols: Tumor Cell Line: RET melanoma cell line, generated by transgenic expression of the Ret protooncogene under the control of the metallothionein-1 promoter driving spontaneous melanogenesis, was provided by Viktor Umansky (DKFZ ��� German Cancer Research Center; syngeneic from C57BL/6J mice). Mice: All animal experiments were carried out in compliance with French and European laws and regulations. The local institutional animal ethics board and French Research Ministery of approved all mouse experiments (permission numbers: 2020_035_25416). Experiments were performed in accordance with government and institutional guidelines and regulations. Female C57BL/6 were purchased from Harlan (France). Mice were used between 7 and 12 weeks of age. All mouse experiments were performed at the animal facility of Gustave Roussy Cancer Campus where animals were housed in specific pathogen-free conditions. Three mice were implanted with 0.5x10^6 RET melanoma cells subcutaneously. Single-cell Sample Process: Ileum samples from C57BL/6 mice with and without RET melanoma were collected and fat tissue, Peyer���s patches and feces were removed. Intestines were cut longitudinally, rolled up and then cut transversally into small pieces into a tube. Pieces were transferred into a new 50-ml tube with 20 ml of intestinal epithelial cells (IEC) medium containing PBS, 5% FCS, 5 mM EDTA and 1 mM DTT. Tubes were vortexed and shaken at 37 ��C for 20 min. Cell suspensions were filtered with a 40 ��m cell strainer into a new tube, centrifuged, resuspended in a solution of PBS 1X (without EDTA and magnesium) containing 0.04% de BSA (400 ��g/ml) and counted. Cell suspensions were stored in LoBind tubes at a concentration of 1.0 x 10 6 cellules/mL and kept on ice until use. Single-cell suspensions were loaded onto a Chromium Single Cell Chip (10x Genomics) according to the manufacturer���s instructions for co-encapsulation with barcoded Gel Beads at a target capture rate of ~10,000 individual cells per sample. Captured mRNAs were barcoded during cDNA synthesis using the Chromium Next GEM Single Cell 3' GEM, Library & Gel Bead Kit v3.1 (10x Genomics) according to the manufacturer���s instructions. The resulting libraries were prepared in parallel in one single batch. We pooled all of the libraries for sequencing in an Illumina flow cell. All the libraries were sequenced with an 8-base index read, a 28-base Read1 containing cell-identifying barcodes and unique molecular identifiers (UMIs), and a 91-base Read2 containing transcript sequences on an Illumina NovaSeq 6000 at Gustave Roussy. Single-cell suspensions were loaded onto a Chromium Single Cell Chip (10x Genomics) according to the manufacturer���s instructions for co-encapsulation with barcoded Gel Beads at a target capture rate of ~10,000 individual cells per sample. Captured mRNAs were barcoded during cDNA synthesis using the Chromium Next GEM Single Cell 3' GEM, Library & Gel Bead Kit v3.1 (10x Genomics) according to the manufacturer���s instructions. The resulting libraries were prepared in parallel in one single batch.