Two-dimensional gel electrophoresis of soluble liver proteins isolated from Cambridge and North Ronaldsay sheep following copper challenge. Proteins were separated in the first dimension on pH 5–8 immobilized pH gradient strips, followed by SDS-PAGE on 12.5% (w/v) gels. Gels were stained with Coomassie Brilliant Blue G-250. Proteins whose expression was statistically significant between both sheep breeds as determined by Students t-test are numbered and the identities of these proteins are given in Tables 1 and 2. The copper level in each individual liver (averaged across dorsal and ventral lobes, μg/g dry weight) is indicated on the top of each panel.