Large datasets have been generated to characterize gene expression and cell types in brain samples from many individuals, but it remains unclear how much inter-individual variability exists in these measures. One paper (part of a package from the US National Institute of Health’s Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative — Cell Census Network (BICCN) published in Science) addressed this question using single-nucleus RNA sequencing and whole-genome sequencing analysis of cortex samples from 75 adult humans. The authors found limited variability in the array of cortical cell types, but substantial variability between individuals in cell type abundance and gene expression. Although the majority of this inter-individual variability remains unexplained, some was associated with disease state, age, sex, ancestry, genetic variation, and technical variables. This study highlights the value of large consortium-based efforts and the need to better characterize the extent of inter-individual variability in molecular profiling studies.