Early Treg expansion is evident following a number of chronic parasitic helminth infections, Strongyloides ratti,43Brugia malayi,44 and Litosomoides sigmodontis,17 and an increase in Treg numbers correlates with increased survival of specific T. muris isolates.20 Expanded Treg numbers are also evident in mice following H. polygyrus infection10, 11, 24 with, as we now demonstrate, a marked selectivity for expansion of Helios+Foxp3+ Tregs in BALB/c mice. Here, FOXP3 is linked to helminthiasis.