Thymic stromal lymphopoietin (TSLP) has recently been identified as an important factor capable of driving dendritic cell maturation and activation. TSLP is a four-helix-bundle cytokine that is expressed mainly by barrier epithelial cells and is a potent activator of several cell types such as myeloid dendritic cells. TSLP 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. Amino acid sequence analysis has shown poor homology between human and mouse TSLP although they exhibit similar biological functions and are expressed in similar tissues. Despite its predicted molecular weight, TSLP often migrates at a higher molecular weight in SDS-PAGE. At least two differentially spliced isoforms of TSLP are known to exist.