Post-translational protein modifications, such as glycosylation or phosphorylation, are the level at which this heterogeneity is expressed, and may serve as disease biomarkers. These modifications only result in small molecular mass changes, meaning that standard size-based separation techniques cannot be used. Isoelectric focusing (IEF) is a powerful technique that can resolve a single electrostatic charge difference between protein isoforms, and can be used for the charge-based separation post-translationally modified proteins require. Multiplexed IEF separations are challenging due to the unique chemical environment needed. Although IEF would be ideal for single-cell protein measurement, current techniques rely on specific antibodies to resolve different isoforms, limiting the assays in scope. Researchers at UC Berkeley have addressed these issues by developing a single-cell isoelectric focusing technique that uses pH gradient arrays. By multiplexing IEF, this technology enables separation of the contents of single cells to get insights into cell heterogeneity while retaining the analytical performance to resolve small charge differences.