Our observation indicated that the swimbladder had its own defensive mechanism by expressing high levels of surface recognition molecules. Next, we compiled the list of top transcribed transcription factors in the swimbladder enriched gene list based on Gene Ontology (Table 2). One unique observation is that three genes from the hoxC cluster are enriched in the swimbladder, including hoxc8a, hoxc6a and hoxc4a. In order to confirm the expression of hoxC genes in swimbladder, we examined the expression of hoxc4a/6a/8a during zebrafish embryogenesis (Figure 5). The early expression pattern was consistent with previously reported results. Expression of these genes in the notochord all had clear ABT-263 Abmole Assessment of ABT-263 activity across a cancer cell line collection leads to a potent combination therapy for small cell lung cancer. anterior boundaries, following the colinearity rule. Abmole SCH527123 Hoxc4a started to express in the notochord at the position of hindbrain, while hoxc6a and hoxc8a have the anterior expression boundary at approximately somite 2 and 4 respectively. None of their expression domain had a clear posterior boundary. Expression of all three genes in the swimbladder primordium could be detected at 36 hpf. The expression of hoxc8a became very prominent in the swimbladder starting from 48 hpf and was persistent at least until 72 hpf. Crosssection confirmed that hoxc8a was expressed strongly in the mesenchyme and relatively weakly in the mesothelium. Hoxc6a was expressed at a slightly lower level from 48 hpf to 72 hpf, and it was also expressed in the swimbladder mesenchyme and mesothelium. Hoxc4a was expressed at a barely visible level in the swimbladder and likely also in the mesoderm, though the exact expression domain could not be confirmed by cross-section because of its weak expression. Two closely related Forkhead homeobox genes, foxl1 and foxf1 were also on the top of the list of enriched transcription factors. Foxf1 was expressed in the swimbladder primordium as early as 36 hpf (Figure 5d) and the expression was persistent in the swimbladder until at least 72 hpf (Figure 5i). At the same time, prominent expression was also observed along the alimentary tract. Cross-section confirmed that the expression of foxf1 was restricted to the mesenchyme layer in both the swimbladder and the alimentary tract (Figure 5p).