  We present a combined experimental and numerical study of the equilibrium
cluster formation in globular protein solutions under no-added salt conditions.
We show that a cluster phase emerges as a result of a competition between a
long-range screened Coulomb repulsion and a short-range attraction. A simple
effective potential, in which only depth and width of the attractive part of
the potential are optimized, accounts in a remarkable way for the wavevector
dependence of the X-ray scattering structure factor.
