Homozygous larvae show disruption of the pattern of innervation and neuromuscular junction structure at muscles 12 and 13. The morphology of the nerve terminals varies, with four categories of phenotype being observed (at least one phenotype is seen in 57% of abdominal segments): overgrowth of nerve terminals to neighbouring muscles ("back branching", 43%), the presence of multiple branch points (11%), defasciculation at the branch point before reaching the muscle (33%), and/or detachment of the nerve branch point from the muscle (45%). Overgrowth and back branching of nerve terminals in muscle 12 to muscle 13 is common. In the multiple branch phenotype, 2 to 4 nerves emerge from the segmental nerve and encounter muscle 12 or 13 at different positions along the muscle fibre. The source of the nerve branch can be hundreds of μm more distal than is normal, resulting in innervation from the lateral, rather than medial aspect of the fibre. Other muscle fibres, such as muscles 1, 2, 3, 4, 19, 20 and 29 also show abnormal nerve terminal morphology, with the most common phenotype being multiple branch points. All the motor terminal types are affected in these mutants; both type I and type II boutons back-branch to muscle 13, and abnormal innervation of muscle 13 by type III motorneurons (which normally predominantly innervate muscle 12) is also seen. The abnormal synaptic phenotype can be seen at all larval stages and is recessive. Besides the defects at the neuromuscular junction, no major morphological abnormalities in the central nervous system or other organs are seen in mod(mdg4)^bpd1 larvae.