Insulin human is a 51 residue peptide hormone, composed of two amino acid chains covalently linked by disulfide bonds. The structure is identical to native human insulin. Recombinant insulin is synthesized by recombinant DNA techncology. Inserting the human insulin gene into the Escherichia coli bacteria or Saccharomyces cerevisiae produces insulin for human use. Inhalable insulin is a powdered form of insulin regular, delivered with a nebulizer into the lungs where it is absorbed. Exubera, developed by Inhale Therapeutics (later named Nektar Therapeutics), became the first inhaled insulin product to be marketed in 2006 by Pfizer, but poor sales led Pfizer to withdraw it in 2007. Afrezza, a monomeric inhaled insulin developed by Mannkind, was approved by the FDA in 2014.