PTN and osteoporosis: Markers for cellular senescence and genes described as mediators of cell cycle were also differentially expressed in these cells, e.g. CDKN2A (P16), several PSG, PTN, ARHGAP29 (PARG1), HMMR and HELLS. Clock-driven aging and osteoporosis showed less negative effects on proliferative capacity of stem cells, but in hMSC-OP the expression of a second well known marker of replicative senescence – besides P16 – was increased: CDKN1A, which codes for P21.