We developed an in vitro system using cytokines interleukin (IL)-3, IL-7, IL-10, and FMS-like tyrosine kinase 3 ligand with CD40 ligand-expressing fibroblasts to obtain single blast colonies from which clonal immunoglobulin heavy chain (IgH), T-cell receptor delta gene rearrangements, and, in t(12;21)-positive cases, TEL-AML1 fusion transcripts could be simultaneously PCR amplified. 