Immortalized myogenic cells expressed p19ARFbut sometimes did not express p16INK4a. Immunoblotting was used to examine p16INK4aand p19ARFexpression by non-immortalized, rapidly growing, Bax-positive myogenic cells (WT Pass 1); NIH3T3 cells; two independently-derived lines of immortalized Bax-null myogenic cells (BaxKO-1 and BaxKO-2); and one line of immortalized Bax-positive myogenic cells (WT-1). Immunoblotting for Bax was used to verify genotypes, and immunoblotting for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was used to verify that similar amounts of protein were analyzed in each lane. As expected , NIH3T3 cells (immortalized mouse embryo fibroblasts) did not express either p16INK4aor p19ARF. Early passage, rapidly growing wild-type cells expressed p19ARFbut not p16INK4a, though p16INK4awas expressed when cells entered "crisis" (not shown). The p19ARFprotein was expressed by all immortalized myogenic cell lines that were examined, whereas the p16INK4aprotein was expressed by some (e.g., BaxKO-2), but not all (BaxKO-1) lines. See text for details.