2-DE reference gel images of male and female mouse kidney specimens from different ages showing protein expression profiles. The equal amounts of solubilised proteins (300 μg) were separated on 11 cm pI 5-8 linear gradient strips in the first dimension and on 12.5% SDS-PAGE in the second dimension. Gels were calibrated for molecular mass (in kDa) and pI (in pI units) with external pI and mass standards and stained with CBB G-250. Proteins with significantly different expression levels between ages are marked with circles and numbered. Numbers refer to protein spot numbers in other figures and additional file 5. (A) gender: male, age: 28 weeks; (B) gender: male, age: 52 weeks; (C) gender: male, age: 76 weeks, (D) gender: female, age: 28 weeks; (E) gender: female, age: 52 weeks; (F) gender: female, age: 76 weeks.