Filgrastim is a recombinant, non-pegylated human granulocyte colony stimulating factor (G-CSF) analogue manufactured by recombinant DNA technology using a strain of E. coli. It is marketed as the brand name Neupogen by Amgen. Chemically, it consists of 175 amino acid residues. The protein has an amino acid sequence that is identical to the natural sequence predicted from human DNA sequence analysis, except for the addition of an N-terminal methionine necessary for expression in E coli. Tbo-filgrastim, which is marketed by Sicor Biotech and FDA approved on August 29, 2012, contains the same active ingredient as Neupogen and is biologically similar, but it is formulated to be short-acting. On March 6, 2015, the FDA approved the biosimilar Zarxio (filgrastim-sndz) and is indicated for use in the same conditions as Neupogen. Zarxio is marketed by Sandoz.