Zeocin is a formulation of phleomycin D1, a basic, water-soluble, copper-chelated glycopeptide isolated from Streptomyces verticillus. The presence of copper gives the solution its blue color. This copper-chelated form is inactive. When the antibiotic enters the cell, the copper cation is reduced from Cu2+ to Cu1+ and removed by sulfhydryl compounds in the cell. Upon removal of the copper, Zeocin is activated and will bind DNA and cleave it, causing cell death.