Fetal lymphoid tissue inducer (LTi) cells are required for the development of secondary lymphoid organs. These cells have been categorized as group 3 innate lymphoid cells (ILC3s) because they express the transcription factor, ROR gamma t, and secrete IL-22. Unlike other ILC3s, however, LTi cells are derived from a different progenitor cell than those that give rise to conventional natural killer cells or the other ILC subsets. Regardless, LTi cells are currently categorized within the heterogeneous ILC3 subset, as (NCR)- ILC3s due to their lack of expression of the natural cytotoxicity receptor, NKp46. Mouse and human LTi cells also express the chemokine receptor, CCR6, along with CD45, CD90/Thy1, CD117/c-kit, and CD127/IL-7 R alpha, which are four cell surface markers that are expressed by all ILC3s. Additionally, LTi cells express Integrin alpha 4 beta 7, CXCR5, and CCR7, and lack expression of the lineage markers (Lin-) that are commonly used to identify other immune cell types. In addition to these markers, human LTi cells also express CD7 and CD161, and lack expression of CD4, while mouse LTi cells can be either CD4+ or CD4-.