A method of preparing canine pancreatic beta cells or canine beta cell tumours, comprising the steps of: a) transducing and co-transducing immature canine pancreas cells with i) a lentiviral vector expressing SV40 LargeT antigen under the control of the insulin promoter, or ii) with a lentiviral vector expressing SV40 LargeT antigen under the control of the insulin promoter and a lentiviral vector expressing hTert under the control of the insulin promoter, or iii) a lentiviral vector expressing both SV40 LargeT antigen and hTert under the control of the insulin promoter; b) introducing the transduced immature pancreas cells obtained in a) into the kidney capsule of a first severe combined immunodeficiency (scid) non-human animal; c) allowing the transduced immature pancreas cells to develop insulinoma-like structures, wherein the immature canine pancreases cells in insulinoma-like structures have differentiated to insulin-producing pancreatic beta cells; d) micro-dissecting the insulinoma-like structures obtained in step c), and dissociating the cells thereof; e) sub-transplanting the cells obtained in step d) into the kidney capsule of a second scid non-human animal; f) allowing the sub-transplanted cells in step e) to develop and regenerate newly developed insulinoma-like structures, wherein said newly developed insulinoma-like structures are enriched in insulin-producing pancreatic beta cells; g) micro-dissecting the insulinoma-like structures obtained in step f), and dissociating and collecting the cells thereof; h) optionally, sub-transplanting the cells obtained in step g) into the kidney capsule of a third non-human scid animal, hence allowing further enrichment and amplification of insulin-producing pancreatic beta cells; and i) optionally repeating step e), f) and g) until the appropriate amount of insulin-producing pancreatic beta cells is obtained. a) transducing and co-transducing immature canine pancreas cells with i) a lentiviral vector expressing SV40 LargeT antigen under the control of the insulin promoter, or ii) with a lentiviral vector expressing SV40 LargeT antigen under the control of the insulin promoter and a lentiviral vector expressing hTert under the control of the insulin promoter, or iii) a lentiviral vector expressing both SV40 LargeT antigen and hTert under the control of the insulin promoter; b) introducing the transduced immature pancreas cells obtained in a) into the kidney capsule of a first severe combined immunodeficiency (scid) non-human animal; c) allowing the transduced immature pancreas cells to develop insulinoma-like structures, wherein the immature canine pancreases cells in insulinoma-like structures have differentiated to insulin-producing pancreatic beta cells; d) micro-dissecting the insulinoma-like structures obtained in step c), and dissociating the cells thereof; e) sub-transplanting the cells obtained in step d) into the kidney capsule of a second scid non-human animal; f) allowing the sub-transplanted cells in step e) to develop and regenerate newly developed insulinoma-like structures, wherein said newly developed insulinoma-like structures are enriched in insulin-producing pancreatic beta cells; g) micro-dissecting the insulinoma-like structures obtained in step f), and dissociating and collecting the cells thereof; h) optionally, sub-transplanting the cells obtained in step g) into the kidney capsule of a third non-human scid animal, hence allowing further enrichment and amplification of insulin-producing pancreatic beta cells; and i) optionally repeating step e), f) and g) until the appropriate amount of insulin-producing pancreatic beta cells is obtained.