Mushroom body calyces in mbr¹ mutants are reduced to approximately 20-50% volume of wild-type flies. Gross brain morphology appears relatively normal.

mbr¹ females are semi-lethal.

mbr¹ heterozygous males demonstrate comparable patterns of landmark orientation, indicating similar responses to visual stimulation in Buridan's paradigm as control flies.

mbr¹ heterozygous males exhibit reduced activity levels and walk slower than control flies.

Flies carrying mbr¹ in a Canton-S background have a small calyx and thin lobes in the adult brain. Long and short latency responses are indistinguishable from wild-type flies in both the DLM (flight) and TTM (jump) muscles. EC₅₀ values (the concentration at which half of the long latency responses are expected to fail) for halothane and enflurane for mutant flies are indistinguishable from the EC₅₀ values of Canton-S controls.

Flies have a number of brain defects, the exact phenotype depending on the genetic background. In the original genetic background in which it was induced, mbr¹ produces the following phenotype; the calyx is 1/4 normal size, the peduncle and lobes are thin, the ellipsoid body is open ventrally and the noduli are usually misshapen. When placed in a Canton S background, the mushroom body and central complex defects are similar to the phenotype in the original genetic background, and the optic lobe lobula is often disorganised.