Nucleating factors are necessary to stimulate actin polymerization. In the latter case, PDLA acts as a nucleating agent, thereby increasing the crystallization rate. Furthermore, it acts as a nucleating agent effecting in a very fine crystalline structure. This zone fills with microtubules nucleating on the surface of the nucleus. It can be used in polymer chemistry as a nucleating agent. Hemolymph can contain nucleating agents that confer extra cellular freezing protection. "Pseudomonas" strains of bacteria cause frost damage by nucleating water into ice crystals around themselves. Rain- and snow-making companies still use silver iodide as a nucleating agent in seeding clouds. Freeze-tolerant insects are known to produce ice nucleating proteins.