JUN and breast cancer: Using an antibody specific for phosphoserine-73, we noted high but variable levels of phosphorylated Jun in extracts from BK5.ATF3 transgenic mammary glands (Figure 2E, lanes 4–6) and from BK5.ATF3-induced mammary tumors (Figure 2E, lanes 7–9); much lower levels of activated Jun were seen in extracts from non-transgenic mammary glands (Figure 2E, lanes 1–3).