In standard molecular biology research, the bacterium "Escherichia coli" is the most frequently used organism for expression system, to produce heterologous proteins, due to its features of fast growth rate, high protein production rate, as well as undemanding growth conditions. Protein production in "E. coli" is usually faster than that in "P. pastoris", with reasons: Competent "E. coli" cells can be stored frozen, and thawed before use, whereas "Pichia" cells have to be produced immediately before use.Expression yields in Pichia vary between different clones, so that a large number of clones has to be screened for protein production, to find the best producer. The biggest advantage of "Pichia" over "E. coli" is that "Pichia" is capable of forming disulfide bonds and glycosylations in proteins, but "E. coli" cannot. "E. coli" might produce a misfolded protein when disulfides are included in final product, leading to inactive or insoluble forms of proteins.