Fig. 2. Identification of the gene responsible for the overexpression phenotype of line 801. (A) Genomic organisation of the region showing the relative positions of different GS and EP elements with respect to Mkp3 and MESR6. EP3142 should only drive MESR6 expression. (B-D) Details of four to five DBs of late stage embryos. Note the defects in continuity of some branches (arrows) when lines GS10283 (B) and Mkp3^M76 (C) are crossed to btlGal4. By contrast, a mutation in the Mkp3 gene prevents the GS element of the Mkp3^M76 line from producing a branch integrity phenotype (D). (E-G) In situ hybridisation with an antisense Mkp3 probe detects accumulation of the transcript in the tracheal placodes at stage 11 (arrow in E) and in the tracheal branches at later stages (arrow in F) in the wild type. Higher levels are detected when line 801 is crossed to btlGal4 (G).