Pediatric solid tumors are rare compared to common adult malignancies and they are also remarkably diverse. For example, rhabdomyosarcomas have features of skeletal muscle, osteosarcomas have features of bone and neuroblastomas have features of cells in the sympathoadrenal lineage. The diversity and rarity of pediatric solid tumors makes it difficult to accelerate biomedical research that can improve patient outcomes. For example, even with the large number of patients treated at St. Jude, it can be difficult to obtain fresh pediatric solid tumor tissue that is suitable for single cell sequencing. To overcome this barrier in the field, we have developed carefully validated methods to process fresh and frozen tissue for single-cell or single-nucleus RNA-sequencing. Our data show that we can capture the transcriptional heterogeneity of the tumors and the complexity of the tumor microenvironment using single-nuclei RNA-sequencing of patient tumors from the St. Jude biorepository. In addition, over the past 9 years, we have generated 166 orthotopic patient derived xenografts (O-PDXs) representing 21 different pediatric solid tumor types. The O-PDXs and corresponding patient tumors have undergone some of the most comprehensive characterization of any pediatric cancer model including detailed analysis of the clonal heterogeneity (Stewart et al. Nature, 2018). We performed single-cell or single-nuclei RNA-sequencing of 17 tissue samples from sarcoma patient tumors and 17 matched O-PDXs as part of a larger effort to perform single-cell/single-nuclei RNA-sequencing on a large cohort of patient tumors and O-PDXs. This research proposal will fill a fundamental gap in our knowledge of the transcriptome heterogeneity across pediatric solid tumor clones and of the normal cells found in the tumor microenvironment. \n\nWe would greatly appreciate if you cite the listed publications when using this data.