Cluster 1 samples had the highest levels of PU.1 expression, and were predominantly M4/M5 FAB classification (myelomonocytic/monoblastic), suggesting a separate class of cases where high PU.1 levels direct a degree of monocytic differentiation, but alternative mechanisms are responsible for the leukaemia phenotype (Figure 6c). The gene discussed is SPI1; the disease is leukemia.