Transcription factor involved in specification of neuronal cell types. Initiates at the cellular blastoderm stage in distinct bilateral domains within the procephalic neuroectoderm. Present in narrow stripes that cover dorsolateral positions at about 75% egg length. During gastrulation, a second bilateral pair appears posteriorly to the initial domains, from which they are separated by a 3 to 4 cell wide gap. During germ band elongation, both the primary and secondary domains of expression become wedge-shaped. During stage 9, a third pair of bilateral domains appears anteriorly to the primary domains. The 3 domains of expression may reflect the proposed subdivision of the protocerebral primordium into an anterior, central, and posterior/optic lobe domain, respectively. The existence of gaps between the domains of expression suggests that it marks only a subset of cells within each of the 3 protocerebral primordia. Following stage 9, neuroblasts segregate from the domains and the central and posterior domains split into smaller subdomains. During later events of brain morphogenesis, the presence of 3 domains of expression can still be discerned, and prominent expression in late stage embryos include the optic lobe region. At late stage 11, it is also expressed in the ventral neuroectoderm, where it appears in a small cluster of neuronal precursors within each hemisegment. When expressed in mice, Hmx is able to functionally replace both Hmx2 and Hmx3 mouse proteins. Belongs to the HMX homeobox family.