Lineage specificity and intron dependency. The fragments protected by the end-labeled S1 probe are shown at the bottom. Total RNA from transfected cell lines was used. (A) Mature B-cell line; (B) pre-B–cell line; (C) non–B-cell (myeloid) line; (D) mature B-cell transfected with the βAPr-Ig▵Introns andβAPr-IgςIntron constructs.