Neuroblastoma is derived from the sympathetic nervous system and might arise as a result of impaired differentiation, retaining the neuroblastic tumor cells in the cell cycle. Thus, to understand the genesis of neuroblastoma, the study of mechanisms and genes regulating normal sympathetic development is of potential interest. The basic helix-loop-helix transcription factors human achaete-scute homolog-1 (HASH-1) and deciduum, heart, autonomic nervous system, and neural crest derivatives (dHAND) are expressed in the sympathetic nervous system of embryonic mice and chicken, with undetectable postnatal expression. By in situ hybridization technique, we show that dHAND was expressed by human sympathetic neuronal and extra-adrenal chromaffin cells throughout embryonic and fetal life, and was initially expressed in immature chromaffin cells of the adrenal gland. With overt chromaffin differentiation, dHAND was down-regulated. HASH-1, in contrast, was expressed in human sympathetic cells only at the earliest embryonic ages examined (Week 6.5 to 7). All examined neuroblastoma specimens (25/25) and all cell lines (5/5) had detectable dHAND mRNA levels. HASH-1 expression in tumor specimens was more restricted, although all cell lines (5/5) were HASH-1­positive. These results show that neuroblastoma tumors have retained embryonic features, suggesting that many neuroblastomas are blocked at an early stage of normal development when HASH-1 and dHAND are expressed. dHAND also appears to be a reliable and potentially useful clinical diagnostic marker for neuroblastoma, because expression was not dependent on tumor or differentiation stages and other pediatric tumors were dHAND-negative. (Lab Invest 1999, 79:67-79).