The present invention relates to a method for preparing commercial scale quantities of human functional Betacells and to the establishment of cell lines. It also relates to a method of diagnosis using Beta cell tumors or cells derived thereof. The method comprises sub-transplantation procedure to enrich the graft in proliferating Betacells, allowing to generate human Betacell lines. Such lines express little amount of insulin and have a gene expression profile that resembles to adult Betacells. In addition, the human Betacell lines are able to normalize glycemia of diabetic mice when transplanted, demonstrating their insulin secretion capabilities.