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RTN1 is an endoplasmic reticulum-associated protein that was initially identified in neuronal tissues. Here we show that the main isoform RTN1A is a marker for dendritic cells. In the skin, HLA-DR+CD1ahighCD207+CD11cweak Langerhans cells were the only cells in the epidermis, and HLA-DR+CD11c+ dendritic cells were the main cells in the dermis, expressing this protein. RTN1A+ dendritic cells were also found in gingiva, trachea, tonsil, thymus, and peripheral blood. During differentiation of MUTZ-3 cells into Langerhans cells, expression of RTN1A mRNA and protein preceded established Langerhans cell markers CD1a and CD207, and RTN1A protein partially co-localized with the endoplasmic reticulum marker protein disulfide isomerase. In line with this observation, we found that RTN1A was expressed by around 80% of Langerhans cell precursors in human embryonic skin. Our findings show that RTN1A is a marker for cells of the dendritic lineage, including Langerhans cells and dermal dendritic cells. This unexpected finding will serve as a starting point for the elucidation of the, until now, elusive functional roles of RTN1A in both the immune and the nervous system.