CD203a, originally clustered as SWC9 at the Second International Swine CD Workshop and later identified as the porcine homologue of human ecto-nucleotidepyrophosphatase /phosphodiesterase 1 or NPP1. CD203a is expressed widely in macrophage populations with notably high levels on alveolar macrophages, it is not expressed on monocyte populations. SWC1a, expressed at very much higher levels on monocytes than mature macrophages and CD203a (SWC9), expressed exclusively on mature tissue macrophages have been used as markers of monocyte-macrophage differentiation.