Thymic stromal lymphopoietin (TSLP) has recently been identified as an important factor capable of driving dendritic cell maturation and activation. It is involved in the positive selection of regulatory T cells, maintenance of peripheral CD4+ T cell homeostasis and the induction of CD4+ T cell-mediated allergic reaction. TSLP is also capable of supporting the growth of fetal liver and adult B cell progenitors and their differentiation to the IgM-positive stage of B cell development. Its receptor TSLP-R will bind TSLP in a low-affinity fashion in transfected cells; cotransfection with IL-7R-alpha cDNA results in high-affinity binding and a functional heteromeric complex. This heteromeric receptor requires stat5 for TSLP-mediated signal transduction and is inhibited by SOCS-1. Despite its predicted molecular weight, TSLP-R often migrates at a higher molecular weight in SDS-PAGE.