James Grellier – derivative of original work by VashiDonsk [creativecommons license] via Wikimedia CommonsSingle-cell analyses have shown that individual cancer cells can acquire mutations that make them more aggressive and resistant to chemotherapy. Thus, in some cases, even if chemotherapy seems to have been effective, a particularly aggressive cancer cell may survive and then cause a relapse in the future. The specific mutations that drive cancer progression and the mechanisms by which they do so are not well understood, delaying improvements in treating chemotherapy-resistant relapse. Researchers from Harvard Medical School and Massachusetts General Hospital (both in Boston, MA) led by David Langenau decided to try a new approach to identify the relapse-driving cells in T-cell acute lymphoblastic leukemia (T-ALL), called leukemia-propagating cells (LPCs), using zebrafish.