  A simple model of globular proteins which incorporates anisotropic
attractions is proposed. It is closely related to models used to model simple
hydrogen-bonding molecules such as water. Theories for both the fluid and solid
phases are presented, and phase diagrams calculated. The model protein exhibits
a fluid-fluid transition which is metastable with respect to the fluid-solid
transition for most values of the model parameters. This is behaviour often
observed for globular proteins. The model offers an explanation of the
difficulty observed in crystallising some globular proteins and suggests that
some proteins may not have a solid phase at all under all but extreme
conditions.
