Purification of anti-HER2

A method for purifying a polypeptide from a composition comprising the polypeptide and contaminants, wherein the polypeptide is an antibody that binds to HER2 and wherein the contaminants comprise a deamidated variety of antibody, and wherein the method comprises the sequential steps of: (a) loading the composition onto a cation exchange resin with an equilibration buffer having a first salt concentration; (B) washing the cation exchange resin with a wash buffer until a predetermined concentration of protein in the flow through, wherein the salt concentration of the wash buffer increases from a second initial salt concentration is increased is measured the salt concentration of equilibration buffer, to a third final salt concentration; (C) passing a fixed volume of 0.4 to 1 column volume of wash buffer to the third final salt concentration on the cation exchange resin; and (d) eluting the polypeptide from the cation exchange resin with elution buffer having a salt concentration which is greater than the final concentration of salt wash buffer.