Frequencies of myeloid, B-, and T-cell lineages. Precursor frequencies within the CD34⁺CD1a^– thymocyte population were determined by limiting dilution assays and calculated using the method of maximum likelihood.⁴³  (Left) CD19⁺ B cells on OP9-GFP; frequency 1 in 644 (720 and 270 in subsequent experiments). (Middle) CD13/33⁺ myeloid cells on OP9-GFP; frequency 1 in 193 (126 in a second experiment) in the absence of GM-CSF (□) and 1 in 48 in the presence of GM-CSF (▦). (Right) CD8⁺ T cells on OP9-DL1; frequency 1 in 29.