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Ontogeny of the Murine Ig Joining Chain Gene and Protein

We used Northern blot analysis in order to investigate the ontogeny of the murine joining (J)‐chain gene. No J‐chain expression was detected in embryonic tissues, including liver, spleen and intestine, but an expression of µ heavy chain was detected in foetal liver at day 17. J‐chain expression was detected in the spleen at day 9 and in the intestine at day 15 after birth. Western blot analysis was carried out in order to compare the protein levels of J and µ heavy chains in serum from day 8 to day 24 after birth, using antihuman J chain and antimouse µ chain antibodies. Although µ chain protein could be detected in serum from day 8, J‐chain protein was detectable only at day 24. These results suggest that the expression of J chain is a later event than the µ chain in the mouse, which thus differs in embryogenesis from humans.