Despite our apparent perfect bilateral symmetric body plan, we all show small but conspicuous left-right asymmetries that are thought to be evolutionarily advantageous. Brain lateralisation, for instance, leads to an increase in cognitive performance. Zebrafish are useful for studying how left-right brain asymmetries arise during embryonic development and how the resultant asymmetric neuronal circuitry ultimately impacts on function and behaviour. This figure is a composition of two intercalated confocal micrographs showing the detail view of the habenular nucleus of a wild-type and a mutant zebrafish embryo. The habenular nucleus is a part of the epithalamus which connects the limbic system (responsible for instinct and mood) to other regions of the brain. Wild-type specimens display noticeable neuroanatomical asymmetries of the habenular nuclei, which can be visualised with the help of fluorescent antibodies labelling different structures in the brain. The arrangement of the habenular axons is shown in magenta and the synaptic neuropil is highlighted in cyan. A subtype of habenular neurons, predominantly enriched in the left side of the brain, express a unique molecular marker and are shown here as green. It is possible to find naturally occurring mutant embryos where this asymmetry has been lost and these are invaluable tools that help form understanding of the molecular mechanisms governing brain lateralization during embryonic development. Width of image is 200 micrometres.