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This paper describes a study of the effect of polarization-dependent loss (PDL) on the signal-to-noise ratio (SNR) in optical communications systems, taking into account the effect of dynamic gain equalization. The paper shows that the PDL-induced SNR penalty is caused by two distinct mechanisms related to the two polarization components of the amplified spontaneous emission noise generated in each amplifier. The noise components whose polarization is parallel to the signal at the various amplifier locations dominate the PDL-induced penalty in the absence of gain equalization. In the presence of even a moderate number of gain equalizers, the orthogonal noise components dominate the SNR degradation. Expressions are obtained relating the outage probability to the necessary performance margin that needs to be allocated for PDL.