Structure of the target and reporter cassettes. In these drawings, the backbone of the vector is represented as a thin line, the cassettes as thick lines, with major exons as rectangles, the LoxP sites as triangles (L1 in the "forward" orientation, 1L in the "reverse" orientation). The three-stranded line represents the chromosomal DNA. The primer sites are indicated by the numbered arrows; numbers refer to the primers listed in Materials and Methods. The figure is not to scale. a) The vector (pH/T) bearing the target cassette. b) The vectors bearing the μ gene replacement cassettes. c) Replacement yielding the μ gene reporter in the forward orientation. d) Replacement yielding the μ gene reporter in the reverse orientation.