The relationship between the mechanical properties of cells and their molecular architecture has been the focus
of extensive research for decades. The cytoskeleton, an internal polymer network, in particular determines a cell’s mechanical
strength and morphology. This cytoskeleton evolves during the normal differentiation of cells, is involved in many cellular
functions, and is characteristically altered in many diseases, including cancer. Here we examine this hypothesized link between
function and elasticity, enabling the distinction between different cells, by using a microfluidic optical stretcher, a two-beam laser
trap optimized to serially deform single suspended cells by optically induced surface forces. In contrast to previous cell elasticity
measurement techniques, statistically relevant numbers of single cells can be measured in rapid succession through microfluidic
delivery, without any modification or contact. We find that optical deformability is sensitive enough to monitor the subtle
changes during the progression of mouse fibroblasts and human breast epithelial cells from normal to cancerous and even
metastatic state. The surprisingly low numbers of cells required for this distinction reflect the tight regulation of the cytoskeleton
by the cell. This suggests using optical deformability as an inherent cell marker for basic cell biological investigation and
diagnosis of disease.