OBJECTIVE To investigate the altered expression of proteins in the lens of mice with inherited cataracts. METHODS Mice with inherited cataracts caused by a spontaneous mutation of the gene gamma S-crystallin (Crygs) were used as the subjects. Lens proteins were extracted and separated by two-dimensional electrophoresis (2-DE). The spots representing differential proteins were first identified by image analysis, and then further analyzed by matrix assisted laser desorption/ionization-time of flight-tandem mass spectrometry (MALDI-TOF-MS/MS). RESULTS 2-DE were conducted under high (882 microg) and low dosage (190 microg) of sample. Under each condition, the numbers of protein spots found in cataract lenses were similar to those in normal lenses (p>0.05). Seventeen proteins were identified in normal lenses, including alphaA- to alphaB-, betaA1- to betaA4-, betaB1- to betaB3-, gammaA- to gammaF-, and gammaS-crystallin, and bead-filament structure protein (BFSP/filensin). Seven differential ones were consistently identified. In the cataract lenses BFSP and gammaS-crystallin were absent; gammaF-crystallin was downregulated; and betaA1-, betaB1-, betaB2-, and alphaB-crystallin were upregulated. Those abnormally upregulated crystallins, when compared to normal ones, had smaller molecular weight, suggesting possible truncation. CONCLUSIONS The mutant Crygs gene can lead to changes of BFSP/filensin and other crystallins. The changes to these crystallins, together, may secondarily lead to cataract formation.