Fetal thymic organ culture revealed the derivation of small CD8⁺TCRβ^low thymocytes from the double-positive CD4⁺CD8⁺ stage of T-cell development. Embryonic day–15.5 thymi of lck-ER^T2-SCL and wild-type littermate control embryos were cultured for 3 and 4 days in the presence of 100 nM hydroxy-tamoxifen (OH-TAM). Representative plots of 3 independent experiments per group are shown. FTOC indicates fetal thymic organ culture; SP, single-positive; and FSC, forward scatter.