Primary expression screening and Southern blotting analysis. (A) A representative ES clone that did not show lacZ reporter expression in undifferentiated ES cells on gelatin but in differentiated ES cells on the OP9 feeder cell layer after 5 days of culture. Scale bars, 50 μm. (B) Southern blotting of genomic DNA prepared from gene-trapped ES cells with a β-gal or neo probe. Approximate size of detected DNA is indicated. Trapped genes: lanes 1-2, not identified; lane 3, endoglin; lane 4, Hes1; and lane 5, ASPP1.