Fig. 1. MAP kinase phosphorylation and expression in nonmalignant and malignant mammary cell lines. Nonmalignant human mammary epithelial cells and a panel of nine human breast cancer lines were grown exponentially in complete growth medium and lysed with RIPA buffer. Twenty µg of lysate protein from each line were fractionated by SDS-PAGE in duplicate gels and immunoblotted with anti-phosphoERK1/ERK2 (A) or anti-ERK2 (B). The anti-ERK2 antibody reacts strongly with ERK2 but less so with ERK1. *, nonmalignant.