Fig. 4. Tethered Twist homodimers rescue some mesodermal defects associated with loss of Twist. (A-E) Wild-type embryos; (F-J) twi^ID96 null mutant embryos; (K-O) twi^ID96 embryos expressing UAS-Twist-Twist. twist null mutant embryos completely lack somatic musculature as shown by Mhc expression (F,G, arrowhead in G) as well as Zfh1-positive pericardial and cardial cells (H; arrowhead), Fas III-positive visceral muscle progenitors (I; arrowhead), and Htl-positive migrating mesoderm (J). Twist-Twist expression in mutant embryos rescued Htl expression during mesoderm induction (O), and somatic muscle formation, although muscle patterning is disrupted (K). Twist-Twist, however, did not rescue either the visceral mesoderm (N; arrowhead) or the heart (M; arrowhead). Instead, multinucleated, Mhc-positive cells appeared dorsally where the heart usually develops (L; arrowhead). Rescue with the Twist monomer or the Twist-Twist homodimer was 100% or 96% respectively.