library_construction_protocol: 106 OVA-RFP E0771 cells were introduced i.v., and 3 days later fluorescent cells were sorted from whole lungs using FACS Aria III Cell Sorter. Cells were collected in the presence of Actinomycin D (Sigma-Aldrich, cat. A1410) in order to minimize ex vivo RNA transcription. RNA was purified from the sorted cells using the QIAGEN Rneasy kit. A second experimental group consisted of 103 OVA- RFP E0771 cells implanted in mammary fat pads to generate primary tumors. Cells from primary tumors were similarly isolated and sorted on day 14 post implantation. Libraries were prepared using a modified SMART-Seq2 (33). RNA lysate clean-up was performed using RNAClean XP beads (Agencourt), followed by reverse transcription with Maxima Reverse Transcriptase (Life Technologies) and wholetranscription amplification (WTA) with KAPA HotStart HIFI 2 × ReadyMix (Kapa Biosystems) for 18 cycles. WTA products were purified with Ampure XP beads (Beckman Coulter), quantified with Qubit dsDNA HS Assay Kit (ThermoFisher) and assessed with a high-sensitivity DNA chip (Agilent). RNA-seq libraries were constructed from purified WTA products using Nextera XT DNA Library Preperation Kit (Illumina). The libraries were sequenced on an Illumina NextSeq 500/550.