We describe a microchip designed to quantify the levels of a dozen cytoplasmic 
and membrane proteins from single cells. We use the platform to assess 
protein-protein interactions associated with the EGF-receptor-mediated PI3K 
signaling pathway. Single-cell sensitivity is achieved by isolating a defined 
number of cells (n = 0-5) in 2 nL volume chambers, each of which is patterned 
with two copies of a miniature antibody array. The cells are lysed on-chip, and 
the levels of released proteins are assayed using the antibody arrays. We 
investigate three isogenic cell lines representing the cancer glioblastoma 
multiforme, at the basal level, under EGF stimulation, and under erlotinib 
inhibition plus EGF stimulation. The measured protein abundances are consistent 
with previous work, and single-cell analysis uniquely reveals single-cell 
heterogeneity, and different types and strengths of protein-protein 
interactions. This platform helps provide a comprehensive picture of altered 
signal transduction networks in tumor cells and provides insight into the effect 
of targeted therapies on protein signaling networks.