GAB1 and embryonal carcinoma: To understand the basis of this difference, we compared Gab1 expression in mESCs with that in other embryonic cell types by western blotting and found that the predominant form of Gab1 protein in ESCs and embryonal carcinoma cells was ∼95 kDa, whereas embryonic fibroblasts and pooled tissues of a mid-gestation embryo possessed the typical 110-115 kDa form (Fig. 1A).