Df(3R)mthl5-AD5/Df(3R)mthl5-AD5 is lethal (all embryos fail to hatch into first instar larvae). Homozygous mutants display significant heart defects, especially with loss of cell-to-cell adhesion between cardioblasts and pericardial cells. Df(3R)mthl5-AD5/+ also show some dissociation of pericardial cells in embryos at low frequencies. Around a quarter of Df(3R)mthl5-AD5/Df(3R)mthl5-AD5 embryos show misaligned cardioblast cells, resulting in abnormally shaped heart tube lumen.