The distal boundary of the "small" insertion seen in 3 patients (no 1–3) fell within a region covered by 3 overlapping BAC clones (CTD-2107G12, RP11-40A7 and RP11-323H21), thus forming a breakpoint cluster (Figure 2) which was estimated to be 280 Kb long and found to house several genes: POMT1, UCK1 and RAPGEF1. In further two samples (patient no 4 and CML-T1) the inserted material was found to be larger (estimated size of 3.9 Mb) and the distal breakpoint fell within a single BAC clone (RP11-92B21) containing the 3' end of the RXRA gene (Figure 2). The gene discussed is RAPGEF1; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.