Prof. Valery Krizhanovsky and his group in the Weizmann Institute’s Molecular Cell Biology Department develop methods for ridding the body of senescent cells, those that have stopped dividing and haven't died, leaving the way for new cells. Recent work by Prof. Valery Krizhanovsky has revealed that a protein called p21 might play a leading role in aging—by mediating the levels of a special class of age-associated cells that accumulate in body tissues. These findings add to Prof. Krizhanovsky’s previous work, in which he advanced our understanding of senescence by developing the first-ever method for counting such cells in tissues. His approach, which involves staining tissues for molecular markers of senescence then imaging them with advanced techniques, revealed a huge jump in senescent cells over time. In young mice, no more than 1% of cells in any given organ were senescent; but in two-year-old mice, the level of senescent cells in some organs rose to 15%.