library_construction_protocol: Single cell suspensions were prepared from the bone metastasis samples immediately upon mouse sacrifice. In brief, bone marrow fluid was rinsed out with PBS from tibias and femurs of bone metastasis sites, and then filtered through 40-µm nylon cell strainers. Hind limbs (tibias and femurs) with bone metastasis were enzymatically dissociated with RMPI-1640 supplemented with 1 mg/ml collagenase IV and 100 U/ml DNase I and incubated at 37°C for 1 h. Then cells were filtered through 40-µm nylon cell strainers, and washed twice with PBS. Red blood cells were removed from single cell suspensions with red blood cell lysis buﬀer and washed twice with PBS. Single cells were then incubated with CD45 MicroBeads to isolate CD45+ immune cells. Single cells were converted to barcoded scRNA-seq libraries through droplet encapsulation, emulsion breakage, mRNA captured beads collection, reverse transcription, cDNA amplification and purification. Indexed sequencing libraries were then constructed according to the manufacturer's protocol. The sequencing library concentration was quantified by Qubit™ ssDNA Assay Kit (Thermo Fisher). DNA nanoballs (DNBs) were loaded into the patterned nano arrays and sequenced on the ultra‐high‐throughput DIPSEQ T1 sequencer using the following read length: 30 bp for read 1, inclusive of 10 bp cell barcode 1, 10 bp cell barcode 2, and 10 bp unique molecular identifier (UMI), 100 bp of transcript sequence for read 2, and 10 bp for sample index.