This study presents a comprehensive analysis of gene expression in human and mouse tissues, organs, and cell lines, aiming to describe the normal mammalian transcriptome. The authors profiled gene expression from 91 samples across a wide range of biological conditions, using high-throughput gene expression arrays. They identified tissue-specific genes, differentially expressed genes, and genes with unknown functions, providing insights into molecular and physiological functions, transcriptional regulation, disease etiology, and comparative genomics. The dataset was integrated with sequence homology-based annotation to enhance gene function prediction. The authors also identified potential disease markers and compared gene expression patterns between mouse and human orthologs, revealing both conserved and divergent functions. A public web resource was developed to facilitate data access and analysis, enabling researchers to explore the transcriptome for biomedical research.This study presents a comprehensive analysis of gene expression in human and mouse tissues, organs, and cell lines, aiming to describe the normal mammalian transcriptome. The authors profiled gene expression from 91 samples across a wide range of biological conditions, using high-throughput gene expression arrays. They identified tissue-specific genes, differentially expressed genes, and genes with unknown functions, providing insights into molecular and physiological functions, transcriptional regulation, disease etiology, and comparative genomics. The dataset was integrated with sequence homology-based annotation to enhance gene function prediction. The authors also identified potential disease markers and compared gene expression patterns between mouse and human orthologs, revealing both conserved and divergent functions. A public web resource was developed to facilitate data access and analysis, enabling researchers to explore the transcriptome for biomedical research.