A screen to identify a drug for administration to a patient in a regenerative drug therapy that regenerates or repairs a tissue or organ in a human body comprises providing a physiologically relevant human progenitor cell and screening the progenitor cell for a factor that induces differentiation into a cell of the tissue or organ. The physiologically relevant progenitor cell is obtained by:- (a) identifying a target phenotype of interest, being a cell of the tissue or organ that is going to be regenerated； (b) obtaining cells of a first type； (c) determining a differentiation protocol which leads to the appearance of the target phenotype from cells of the first type via a progenitor cell； and (d) modifying the protocol such that the differentiation process is stalled at a stage in which the progenitor cells are present, thereby providing said physiologically relevant progenitor cells for use in the screen.
