Sequence-specific transcription factor which is part of a developmental regulatory system that provides cells with specific positional identities on the anterior-posterior axis. First expressed at 80-100% epiboly with an anterior limit at the presumptive rhombomere 4 (r4) of the developing hindbrain. Continuous expression in, and posterior to r4 until the 1-somite stage. By the 3-somite stage, expression is up-regulated in r4 and down-regulated in r5 and r6. At the 10-somite stage, weak expression in and posterior to r7. By the 15-somite stage, expressed only in r4. Belongs to the Antp homeobox family. Labial subfamily.