Recently, we applied smFISH to quantify the intra-tumor transcriptional heterogeneity of the oncogenic fusion gene, BCR-ABL, in chronic myeloid leukemia [13], and to study the cell-to-cell variability of immunoglobulin gene expression in follicular lymphoma cell lines [14]. Here, BCR is linked to chronic myelogenous leukemia, BCR-ABL1 positive.