Crystallization of the Euplotes raikovi mating pheromone Er-1

A protein mating pheromone E r -1 from the ciliate Euplotes raikovi has been crystallized from (NH 4 ) 2 SO 4 in two forms. Both are suitable for structural studies to at least 2·8 A resolution. Both unit cell sizes are consistent with a tetramer of molecular weight 17,640 in the asymmetric unit.