Retinoic acid alters hindbrain Hox code and induces transformation of rhombomeres 2/3 into a 4/5 identity

The development of the hindbrain in fetuses is affected by retinoic acid, due to its influence on the expression of Hox genes. Experiments with transgenic mice show that the anatomical and molecular changes to the hindbrain and branchial region of the head are due to changes in the Hox code, induced by retinoic acid. These changes transform rhombomeres (r)2/3 homeotically to an r4/5 identity. The changes extend to several parts of the first branchial arch.