Cellular differentiation can be characterized by the acquisition of specified properties during several steps of development whereby the original stem- or precursor-like populations can finally obtain a certain phenotype with highly specific cell functions. The continuing maturation process can be paralleled by progressively reduced proliferative capacity in various cell types functioning as postmitotic tissues. Conversely, other cell populations (e.g., distinct immune cells) may carry out their specific function upon stimulation of proliferation. While these differentiated phenotypes perform their appropriate specific duties throughout the functioning organism, nature may provide an interesting alternative within this concept of life: sometimes, differentiation steps appear to be reversible. Thus, retrograde differentiation--also termed retrodifferentiation--and accordingly rejuvenation may occur when differentiated cells lose their specific properties acquired during previous steps of maturation. Consequently, retrodifferentiation and rejuvenation could provide enormous potential for tissue repair and cell renewal; however, regulatory dysfunctions within these retrograde developments may also involve the risk of tumor promotion.