May be involved in signal transduction as a component of a multimeric receptor complex. Ubiquitous expression in normal tissues. Expression is more elevated in adult liver, lung, spleen, and heart than in their fetal counterparts, and is higher in normal tissues than in the cancerous tissue or cell lines. Low levels of expression were detected in the promonocytic stage, whereas high levels of expression were detected in mature monocytes. Belongs to the MS4A family.