Gel-shift analyses of the sequence at the exon 1/intron 1 border. Raji nuclear extracts were prepared and tested for their ability to shift three γ³²P-labeled oligonucleotides by EMSA. (A) The normal −5+16 probe and the mutant −5+16M probe, indicated above their respective lanes, were incubated in the absence or presence of 200× excess unlabeled oligonucleotide, shown below each lane. (B) The +12+32 labeled probe, indicated above the lanes, was incubated in the absence or presence of 200× excess cold probe as shown below. The three arrows indicate the observed gel-shifts.