Hypercellular nonhuman organisms have functionally inactivated expression of a cyclin inhibitor gene, especially p27. The growth rate of nonhuman organisms are increased such that a desired size is attained more quickly than as compared to nonvariant organisms. Inhibitors of the p27 cyclin dependent kinase inhibitor protein or sequences encoding the protein modulate vertebrate cell cycle progression and increase the proportion of dividing cells to non-dividing cells in a population of treated cells. As the proportion of dividing cells increases, the cell population, e.g., hematopoietic progenitor (stem) cells, is more efficiently used for gene therapy applications. Transgenic animals and plants, and knockout alleles are provided.