Second, in terms of single-cell analysis, we found that varying proportions of basic cell types (with different expression signatures) accounted for a large fraction of the expression variation across a population of individuals. However, this assumes that the expression levels characterizing a signature are fairly constant over a population of cells of a given cell type. In the future, larger-scale single-cell studies will allow us to examine this question in detail, perhaps quantifying and bounding environment-associated transcriptional variability. In addition, current single-cell techniques suffer from low sensitivity and dropouts; thus, it remains challenging to reliably quantify low-abundance transcripts (15, 53). This is particularly the case for specific brain cell substructures, such as axons and dendrites (15).
