CSR junctions in Iμ-Cμ intron–deleted mice and potential cryptic splice site in Iμ exon. (A) Localization of switch donor sites. The donor sites fall into 2 groups, group A (▴) comprises the switch donor sites within Iμ and group B (▵) those within the mutated Iμ-Cμ intron. The relative position of cEμf primer is indicated. cEμ indicates the core Eμ enhancer; H, HindIII; R, EcoRI; and X, XbaI. (B) Putative cryptic splice site in Iμ exon. (Top) Relative position of the primers used in RT-PCR. (Bottom) The first line shows the localization of the putative cryptic donor splice site (underlined) downstream of the octamer motif of the core Eμ enhancer. Three transcription initiation sites⁵²  are indicated by asterisks. The second line shows the correctly spliced shortened Iμ (Iμ*) exon to Cμ exon. The third line shows the nucleotide sequence in the vicinity of Cμ acceptor splice site (underlined).