FOXN1 and thymus atrophy: Moreover, in mice expressing a FOXN1 hypomorph64 we found that accelerated emergence of aaTECs accompanies early thymic atrophy; however, although our velocity analysis suggests that aaTECs are the downstream products of differentiation from mTEC1s and earlyprog and may represent end-stage epithelial cells arising and accumulating in the involuted thymus, an alternative hypothesis could be that aaTECs are progenitor cells that have become blocked.