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Lasers made of thin layers of semiconductor materials have enabled many new technologies, including the compact-disk player. The authors have tried something new with these tiny lasers: inserting living cells into the laser cavity in order to "read" the cells’ characteristics in the way they alter the spectrum of emitted light. At certain wavelengths cells become transparent, revealing their composition by variations in the refractive index. These variations provide spectral information that distinguishes, for example, malignant or anemic cells from normal cells. Gourley and Sasaki tell a story that involves problems in human medicine, advances in solid-state physics and sticky problems in surface chemistry and microfabrication.