Fig. 6. Model of cell cycle progression by adult newt cardiomyocytes in culture. This incorporates, into a diagram, the various results described in the text. This figure should be seen only as a model based on the data, since it incorporates data from different experiments (time lapse analysis; dose response assay for FBS; Rb phosphorylation and inhibition of re-entry by p16^INK4). Note that all percentages are given with reference to the starting population of cardiomyocytes. Although our data does not rule out the possibility that another member of the pocket-protein family may also mediate the regulation of cell cycle entry, to date no other members of this family have been cloned in amphibia.