Hierarchical cluster of probe sets regulated in zebrafish embryos following chronic morphine exposure. Raw data from six independent hybridizations are presented for the two experimental groups: control and morphine-treated embryos zebrafish. The probe-set selection was based on a standard statistical analysis with Robust Multi-array Analysis (see Methods for details). Hierarchical cluster analysis was performed using Cluster 3.0 and Treeview software and the tree image represents low (in blue) or high expression (in red) of probe sets. These probe sets represent 87 genes (representing 93 probe sets) involved in Neuronal Function (a complete list is provided in Additional file 4). The twelve genes (fourteen probe sets) chosen for validation of microarray data by RT-qPCR are listed (right hand side of the figure).