Retinoic acid (RA) can induce growth arrest and neuronal differentiation of 
neuroblastoma cells and has been used in clinic for treatment of neuroblastoma. 
It has been reported that RA induces the expression of several HOXD genes in 
human neuroblastoma cell lines, but their roles in RA action are largely 
unknown. The HOXD cluster contains nine genes (HOXD1, HOXD3, HOXD4, and 
HOXD8-13) that are positioned sequentially from 3' to 5', with HOXD1 at the 3' 
end and HOXD13 the 5' end. Here we show that all HOXD genes are induced by RA in 
the human neuroblastoma BE(2)-C cells, with the genes located at the 3' end 
being activated generally earlier than those positioned more 5' within the 
cluster. Individual induction of HOXD8, HOXD9, HOXD10 or HOXD12 is sufficient to 
induce both growth arrest and neuronal differentiation, which is associated with 
downregulation of cell cycle-promoting genes and upregulation of neuronal 
differentiation genes. However, induction of other HOXD genes either has no 
effect (HOXD1) or has partial effects (HOXD3, HOXD4, HOXD11 and HOXD13) on 
BE(2)-C cell proliferation or differentiation. We further show that knockdown of 
HOXD8 expression, but not that of HOXD9 expression, significantly inhibits the 
differentiation-inducing activity of RA. HOXD8 directly activates the 
transcription of HOXC9, a key effector of RA action in neuroblastoma cells. 
These findings highlight the distinct functions of HOXD genes in RA induction of 
neuroblastoma cell differentiation.