Sir

I would like to correct an error in your Opinion article “Systems Biology's Multiple Maths” (Nature 407 , 819; 2000) about the approach and the scope of the Alliance for Cellular Signaling. Our focus is limited to G protein-regulated and related (interacting) cellular signalling systems, not “every protein–protein interaction in two cell types”. We seek quantitative information about these systems at all levels — not just detection of interactions between proteins. Further details can be found on our website, http://afcs.swmed.edu; see also Nature 402, 219; 1999.