A survey of human brain transcriptome diversity at the single cell level reveals the complexity of the adult and fetal human brain at the whole transcriptome level using single cell RNA sequencing. The study analyzed 466 cells from healthy adult temporal lobe tissue obtained during surgical procedures for patients with refractory seizures. The researchers classified individual cells into major neuronal, glial, and vascular cell types, identifying seven neuronal subcategories, including excitatory and inhibitory neurons. They also identified genes differentially expressed between fetal and adult neurons and observed the expression of major histocompatibility complex type I genes in a subset of adult neurons, but not fetal neurons. The study demonstrated the applicability of single cell RNA sequencing to the study of the adult human brain and constitutes a first step toward a comprehensive cellular atlas of the human brain. The research highlights the molecular heterogeneity of the human brain, showing that the adult human brain is composed of multiple cell classes, each with distinct subtypes based on function, topology, and molecular characteristics. The study also found that the expression of certain genes, such as PVALB, is lower in humans compared to mice, suggesting interspecies differences. The results show that single cell RNA sequencing can be used to identify cell types in the brain without prior selection of genes, with high accuracy. The study also identified differences in cell-type prevalence and expression patterns between mice and humans. The findings contribute to a better understanding of the molecular diversity of the human brain and provide a foundation for future research on the cellular composition of the human brain.A survey of human brain transcriptome diversity at the single cell level reveals the complexity of the adult and fetal human brain at the whole transcriptome level using single cell RNA sequencing. The study analyzed 466 cells from healthy adult temporal lobe tissue obtained during surgical procedures for patients with refractory seizures. The researchers classified individual cells into major neuronal, glial, and vascular cell types, identifying seven neuronal subcategories, including excitatory and inhibitory neurons. They also identified genes differentially expressed between fetal and adult neurons and observed the expression of major histocompatibility complex type I genes in a subset of adult neurons, but not fetal neurons. The study demonstrated the applicability of single cell RNA sequencing to the study of the adult human brain and constitutes a first step toward a comprehensive cellular atlas of the human brain. The research highlights the molecular heterogeneity of the human brain, showing that the adult human brain is composed of multiple cell classes, each with distinct subtypes based on function, topology, and molecular characteristics. The study also found that the expression of certain genes, such as PVALB, is lower in humans compared to mice, suggesting interspecies differences. The results show that single cell RNA sequencing can be used to identify cell types in the brain without prior selection of genes, with high accuracy. The study also identified differences in cell-type prevalence and expression patterns between mice and humans. The findings contribute to a better understanding of the molecular diversity of the human brain and provide a foundation for future research on the cellular composition of the human brain.