hFIX expression vectors. pLIXSN contains a hFIX cDNA driven by long terminal repeat (LTR) promoter. pdLMe4βAhIXm1 contains a hFIX minigene (hIXm1) under the transcriptional control of β-actin promoter (βA) and four copies of MCK enhancer (Me4). I to VIII indicates eight exons of hFIX gene. SVNeo indicates a neomycin resistant gene under the control of SV40 immediate early promoter. X in LTR indicates the deleted enhancer region. Thin lines with arrow show the predicted mRNA species; and zigzag line indicates the portion (hFIX intron) to be spliced.