Fig. 9. The relationship between Fgf signaling and Hoxa2. (A) Fgfs act on target cells to elicit a response (gray dots in the square). Active Fgf signaling blocks Hoxa2 expression; conversely, Hoxa2 blocks Fgf signaling. (B) When neural crest cells expressing high amounts of Hoxa2 are exposed to Fgfs, no response to the signal is obtained (white box); these cells remain Hoxa2 positive and Fgf unresponsive. (C) When neural crest cells expressing low (or no) amounts of Hoxa2 are exposed to Fgfs, responses to the signal are obtained (gray box). These signals will further reduce the Hoxa2 levels, thus increasing the responsiveness of the cells to Fgfs (black box). These cells will turn Hoxa2 negative and Fgf responsive.