Transgenic constructs and copy number. Schematic map of mouse IgH locus (top) with variable region exons (V, D, J) shown as □ and C region genes as ▪. Small ovals with arrows represent GL promoters, and larger ovals correspond to IgH locus enhancers. The 3 transgenic constructs are depicted below. The exons in transgenic constructs are represented as ⊡, and the tag sequence in CH1 is shown as ▧. The spliced transgenic transcript is shown below the EPS-ISC construct. The locations of RT-PCR primers used for Iϵ-CH1 and Iϵ-tag PCRs are indicated by arrows.