Location of the trapped exon. Upper panel shows the structure of proviral Akv MLV DNA with the positions of the splice sites indicated (SD; splice donor, SA; splice acceptor). Arrows signify the PCR primers used to verify the stability of the introduced mutations. Lower panel shows the positions and types of the introduced mutations, marked by asterisks and underlined. The SA'/SD'-delineated exon is indicated by the box. The boldfaced A in the sequence indicates the presumed branch point.