The mushroom bodies are bilaterally arranged structures in the protocerebrum of Drosophila and most other insect species. Mutants with altered mushroom body structure have been instrumental not only in establishing their role in distinct behavioral functions but also in identifying the molecular pathways that control mushroom body development. The mushroom body miniature 1 (mbm 1 ) mutation results in grossly reduced mushroom bodies and odor learning deficits in females. With a survey of genomic rescue constructs, we have pinpointed mbm 1 to a single transcription unit and identified a single nucleotide exchange in the 5 untranslated region of the corresponding transcript resulting in a reduced expression of the protein. A daptive behavior of animals and humans requires functional neuronal circuits in the brain. The genetic programs that control the generation of these circuits by providing an adequate number of neurons, establishing neuronal connectivity, and remodeling them during development and in response to external stimuli during adulthood are just beginning to emerge. The mushroom bodies (MBs), a prominent neuropil structure of the insect brain (1), have become an attractive model system to study many aspects of this intricate network. Functional studies have established a role of the MBs in olfactory learning and memory, controlling locomotor activity, performing visual context generalization, and decision making (2, 3). On the other hand, the structural organization and the development of the MBs have been described in great detail in refs. 4-10. In the adult fly Drosophila melanogaster, Ï·2,500 intrinsic neurons (Kenyon cells) build up one MB. The Kenyon cell bodies are located in the dorsal cortex and extend their dendritic branches into the calyx, where prominent inputs from other brain regions are received. The Kenyon cell axons fasciculate in the peduncle and extend rostroventral, where most of them bifurcate to form a system of medially and dorsally projecting lobes. Each MB arises from a group of four apparently equipotent neuronal stem cells (neuroblasts), each of which generates in a sequential manner several types of Kenyon cells during larval and pupal stages. MB â„ neurons are born before the mid-third-larval instar, then âŁĐÍâ€Đ neurons are born, and finally the âŁÍ†neurons are added at pupal stages (6,8). The nomenclature of the Kenyon cells refers to the corresponding dorsally and medially projecting MB lobes in the adult fly (see Fig. 2). More recently, immunohistochemical studies have identified additional subtypes of Kenyon cells (9). Yet, the anatomical description disregards the structural plasticity of the adult MBs as a consequence of changes in living conditions and experience (11). Furthermore, Kenyon cell axons and dendrites undergo massive remodeling during metamorphosis to establish adult-specific branching patterns. The axons of the â„ neurons, which bifurcate in a dorsal and a medial branch in the larvae, degenerate and regrow only in the medial direction (8,10,(12)(13)(14).What...