The AFM measurements (Fig. 2 e) show a significant decrease in the contact modulus after the addition of Y-27632 (p < 10À6). This apparent discrepancy with the SAM data may appear surprising, but we will show that this is consistent with the tensorial representation of the elastic properties of cells. Note that the statistical scatter of the two measurements are quite different, with the acoustic wave velocity measurement showing a much lower variation as a fraction of the measurement value than the corresponding AFM data. We interpret this as demonstrating a greater intrinsic variability in the AFM measurement of cell stiffness. This is almost certainly caused by error in identifying the contact between the AFM tip and the cell surface, which will lead to uncertainties in the precise value of d in Eq. 3a (50). When the tip is immersed, there are further hydrodynamic effects that may additionally