The sensitivity limit for one-lane DNA sequencing based on laser-induced fluorescence is derived by statistical means. A closed-form expression for the sequencing error probability as a function of the signal strength and of the differences in the fluorescence properties of the labels is given. Fluctuations of the signal caused by photodestruction of the molecules cannot be neglected. The expected sequencing error probability is expressed in terms of the number of molecules and the physical parameters of the measurement. This equation can be used in many ways to design and optimize sequencing experiments.