A method of production of human insulin producing cells, the method comprising the steps of: (a) seeding a multi-tray system with a seeding dose of primary human adult liver cells; (b) propagating and expanding the cells of step (a) to at least about 1 × 10 9 cells in a bioreactor; (c) transdifferentiating the cells of step (b), wherein the transdifferentiating comprises: infecting the expanded cells with a first expression vector, (i) wherein said first expression vector comprises an adenoviral vector comprising a nucleic acid encoding a PDX-1 polypeptide and an adenoviral vector comprising a nucleic acid encoding a second pancreatic transcription factor polypeptide selected from NeuroD1 and Pax4, wherein the infecting with said adenoviral vectors occurs at the same time, or (ii) wherein said first expression vector comprises an adenoviral vector comprising a nucleic acid encoding a PDX-1 polypeptide and a second pancreatic transcription factor polypeptide selected from NeuroD1 and Pax4; and infecting the expanded cells of with a second expression vector, said second expression vector comprising an adenoviral vector comprising a nucleic acid encoding a MafA polypeptide, and (d) harvesting said transdifferentiated cells. (a) seeding a multi-tray system with a seeding dose of primary human adult liver cells; (b) propagating and expanding the cells of step (a) to at least about 1 × 10 9 cells in a bioreactor; (c) transdifferentiating the cells of step (b), wherein the transdifferentiating comprises: infecting the expanded cells with a first expression vector, (i) wherein said first expression vector comprises an adenoviral vector comprising a nucleic acid encoding a PDX-1 polypeptide and an adenoviral vector comprising a nucleic acid encoding a second pancreatic transcription factor polypeptide selected from NeuroD1 and Pax4, wherein the infecting with said adenoviral vectors occurs at the same time, or (ii) wherein said first expression vector comprises an adenoviral vector comprising a nucleic acid encoding a PDX-1 polypeptide and a second pancreatic transcription factor polypeptide selected from NeuroD1 and Pax4; and infecting the expanded cells of with a second expression vector, said second expression vector comprising an adenoviral vector comprising a nucleic acid encoding a MafA polypeptide, and infecting the expanded cells with a first expression vector, (i) wherein said first expression vector comprises an adenoviral vector comprising a nucleic acid encoding a PDX-1 polypeptide and an adenoviral vector comprising a nucleic acid encoding a second pancreatic transcription factor polypeptide selected from NeuroD1 and Pax4, wherein the infecting with said adenoviral vectors occurs at the same time, or (ii) wherein said first expression vector comprises an adenoviral vector comprising a nucleic acid encoding a PDX-1 polypeptide and a second pancreatic transcription factor polypeptide selected from NeuroD1 and Pax4; and (i) wherein said first expression vector comprises an adenoviral vector comprising a nucleic acid encoding a PDX-1 polypeptide and an adenoviral vector comprising a nucleic acid encoding a second pancreatic transcription factor polypeptide selected from NeuroD1 and Pax4, wherein the infecting with said adenoviral vectors occurs at the same time, or (ii) wherein said first expression vector comprises an adenoviral vector comprising a nucleic acid encoding a PDX-1 polypeptide and a second pancreatic transcription factor polypeptide selected from NeuroD1 and Pax4; and infecting the expanded cells of with a second expression vector, said second expression vector comprising an adenoviral vector comprising a nucleic acid encoding a MafA polypeptide, and (d) harvesting said transdifferentiated cells.