ERK1 expression inhibits Ras-dependent tumor formation in nude mice. (a) NIH 3T3 clones were transfected as indicated and expression of the relevant transgenes assessed by western blotting. (b) Growth of tumors in injected nude mice was monitored over 6 days starting from day 4 after injection, by determining the skin area covered by the tumor mass (mm2). The data are expressed as mean ± SEM of two independent experiments (ten animals per clone). (c) Representative tumors after sacrifice at day 10 are shown. (d) Mean weight (± SEM) of the different tumor samples is indicated. Asterisk indicates a genotype effect significant at p < 0.001, calculated using a post-hoc comparison in one-way ANOVA (Scheffe's test: RasQ61L-ERK1 versus RasQ61L-pMex; RasQ61L-ERK1K72R versus RasQ61L-pMex).