The presence of the ER proteins grp78/BiP, grp94 and PDIA6 in milk of a-casein deficient mice is unexpected. We therefore analysed whether the grp78/BiP, grp94 and PDIA6 proteins are up-regulated in lactating mammary tissue of a-casein null mice using protein gel analysis (Fig. 4a) and Western blotting (Fig. 4c) . In total protein extracts derived from mammary tissue it is evident that grp78/BiP and grp94 expression is indeed up-regulated in response to the absence of a-casein. In contrast, the PDIA6 protein can not discerned in Coomassie stained gels of mammary protein extracts, as it co-localises with other proteins of similar molecular mass. Grp78, grp94 and PDIA6 are all ER resident The number of amino acids (aa) and the expected molecular weight (MW) are given for the mature proteins (i.e. without the signal peptide) where appropriate. In addition, the reported molecular weights for the mouse caseins [28] and the molecular weights observed in the SDS-PAGE analysis are shown. The MS score is a measure of confidence of the detected protein species. A score higher than 39 is significant. The number of peptide sequences and the fraction of the total protein covered are also indicated. Protein bands #1-8 are the predominant protein species in the milk of wild-type mice. Protein bands #9-13 are detected in the milk of acasein deficient mice but not in the milk of wild-type or heterozygous mice. L-AAO-1: L-amino acid-oxidase1, PDIA6: protein disulfide isomerase associated 6. doi:10.1371/journal.pone.0021775.t004