Novel aspects of tumor biology and translational approaches using zebrafish models. (A) Studying clonal evolution in T-cell acute lymphoblastic leukemia (T-ALL) using zebrafish: syngeneic zebrafish were engrafted with single, fluorescently labeled clones isolated from primary Myc-induced T-ALL. Individual clones show functional variation, with a minority of clones enhancing growth rate and leukemia-propagating potential. Protein kinase B (AKT) pathway was acquired in a subset of the clones, and increased the latency, the frequency of relapse due to the presence of leukemia-propagating cells (LPCs) and the resistance to chemotherapy in the absence of drug exposure. The study by Langenau and colleagues suggests that a combination of dexamethasone and AKT inhibitors could block tumor relapse [34]. (B) Studying the immunological phenotype of melanoma: nodular tumor from transgenic zebrafish were cryosectioned and analyzed by immunohistochemistry for CD4-1+ lymphocytes. Using this model Hurlstone and colleagues identified different lesions: tumor with extensive lymphocytes infiltration, little infiltration or no infiltration [55]. Quantitative (q) PCR analysis of CD4-1+ tumor infiltrating lymphocytes (TILs) reveals high levels of gata3 and Th2-associated cytokine il-4/13a but not il-4/13b. This study suggests that tumor immuno responses can be faithfully studied in zebrafish models. (C) Studying new ways to deliver chemotherapeutics across the blood-brain barrier (BBB): exosomes were isolated from brain endothelial cells (bEND.3) and cultured in exosome-depleted FBS Media. Fluorescently labeled vascular endothelial growth factor (VEGF) small interfering RNA (siRNA) was loaded in exosomes by lipofectamine. Five days post fertilization (dpf) Tg(fli1a:GFP) zebrafish were injected with exosomes-siRNA into the common cardinal vein. Exosomes containing fluorescent siRNA crossed the BBB and were delivered in the brain, demonstrating that exosomes could be valid carriers of drugs across the BBB and could be studied in zebrafish [56]. (D) Using zebrafish to study endocrine tumors: zebrafish thyroid tumors harbor a gene expression signature that stratifies disease recurrence in patients with papillary thyroid cancer (PTC), identifying patients with significant lower risk of relapse. Twist3, the ortholog of the human Twist2 was found upregulated in BRAF^V600E-expressing thyrocytes. Loss of twist3 expression using Crispr/Cas9 technology demonstrated the key role of this transcription factor in thyroid cancer initiation [57].