Interleukin 9 (IL9) is involved in mast cell maturation and the enhancement of IgE production by B cells. Furthermore, linkage data in human and mice have suggested that IL9 may contribute to asthma. IL9R is known to be autosomal in mice, and is X-linked only in primates.The more recent X linkage and more telomeric position of the IL9R gene may explain its autosomal, 'uninactivated' transcriptional status.