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gbx1 expression is repressed by hoxb1a in r4. a gbx1 expression is restored in hoxb1a mutant embryos. Embryos from a cross of hoxb1a heterozygous parents were assayed by ISH for gbx1 expression, producing two phenotypes (i, ii). Subsequent genotyping revealed that homozygous hoxb1a mutants express gbx1 in r4 (ii, iv). b gbx1 is initially expressed in r4 (iv), but disappears (v, vi) when hoxb1a expression is activated (i, ii, iii). c A mixture of hoxb1a and GFP mRNA was injected into 1-cell stage embryos and successfully injected embryos (identified by GFP expression) were stained for gbx1 expression at 24hpf. The observed reduction in gbx1 expression demonstrates that hoxb1a is capable of repressing gbx1 expression. d Hypothetical model depicting the potential relationship between gbx1 and hoxb1a. hoxb1a could either repress gbx1 directly (solid red T bar) or indirectly by activating a repressor (X; blue arrow) or repressing an activator (Y; orange T bar)
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