B94 was originally described as a novel tumor necrosis factor-alpha-inducible primary response gene in endothelial cells which was also induced in an in vitro model of angiogenesis. To further characterize its expression, we cloned the mouse homologue and mapped its developmental and tissue specific expression. The predicted amino acid sequence of mouse B94 was found to be 83% similar to its human homologue. The gene was localized to mouse chromosome 12 just centromeric to the immunoglobulin heavy chain locus, in a region that is often rearranged in T-cell neoplasms. To explore the possibility that B94 is expressed during vasculogenesis and other developmental processes, the expression of its transcript was determined during mouse development by in situ hybridization. In 10-day embryos B94 was expressed prominently in the myocardium and in the aortic arch. By the 15th day of gestation, expression was restricted largely to the liver, the bone forming regions of the jaw, the aortic endothelium, and the nasopharynx: a pattern that was maintained until just prior to birth. Postnatally, expression shifted to the red pulp of the spleen and the thymic medulla. B94 expression was extinguished in most adult tissues but was detectable in lymphopoietic tissues including the spleen, tonsil, and lymphatic aggregates in the gut. Consistent with this was the finding that mononuclear progenitor cells in bone marrow and mature peripheral blood monocytes expressed B94. A truncated testis-specific transcript previously identified by Northern blot analysis was determined to result from the use of an alternate polyadenylation signal which was surprisingly located within the open reading frame. This shorter transcript was expressed at high levels exclusively in late stage spermatids. Immunostaining with an affinity-purified polyclonal antiserum revealed B94 to be localized to the acrosomal compartment of mature sperm. These studies demonstrate that B94 expression is tightly regulated during development and suggests distinct roles for B94 in myelopoiesis and spermatogenesis.