222 A. Jögi et al. tumor types including breast cancer (9). Increased tumor protein levels of HIF-1 and HIF-2, respec- tively, are also linked to poor patient outcome in breast cancer (reviewed in (26)). In particular, we have shown that HIF-2 protein accumulation corre- lates to worse breast cancer speciﬁc survival and distant metastasis (26). Formation of distant metas- tases has been attributed to the presence of cancer stem or tumor-initiating cells, and we speculate that HIF-2 is a marker of such cell populations. The nature of the breast cancer stem cell is still far from established; the CD44+/CD24- phenotype reported by Al-Hajj et al. (27) does not seem to be the exclusive breast tumor-initiating cell totem. It has not been established that breast tumor-initiating cells have breast stem cell phenotype, and it has lately been implied that stem cell-like cancer cells can arise from more differentiated cells, e.g. through the pro- cess of EMT (28). Hypoxia has been shown to induce EMT in tumors (29) and may be one process wherein breast cancer cells with stem cell properties arise. This general differentiation-counteracting effect of hypoxia has direct bearing on tumor aggressiveness, as tumors with immature features are more aggressive than the corresponding differentiated tumors. Tumor cell differentiation/ dedifferentiation pathways in relation to normal development Tumor cells recapitulate morphology and central gene expression proﬁles of non-malignant cells, which is the basis for histopathological and immunohisto- chemical classiﬁcation of solid tumors. The differen- tiation traits, apparent or delicate, together with location of the primary tumor, determine the histo- pathological diagnosis. Both at the morphological and molecular levels, the degree or stage of differentiation of the tumor cells is estimated, and depending on the outcome of these estimations tumors are often viewed as being arrested at early or late stages of differenti- ation. However, whether a given tumor cell differen- tiation stage reﬂects a distinct stage during normal development has to our knowledge not been studied in any detail. Based on our own neuroblastoma data, we would claim that this is not the case. While normally developing human sympathetic neuro- blasts appear to express a given set of genes in a co-ordinated and repeated fashion between embryos (30), only a subset of the same set of genes is usually expressed in a given tumor, and this subset can differ from one tumor to another and between tumor cells within a tumor as exempliﬁed by the neuron-speciﬁc enolase (ENO2) expression (in neuroblastoma and ganglioma, the differentiated form of neuroblastoma) (Figure 2) (31). Thus, we would like to claim that differentiation pathways are disorganized in neuroblastoma and that this aberrant differentiation is a tumor characteristic. We hypothesize that this might also be the case in many other tumor forms. Similarly, when tumor cells dedifferentiate, do they then play back and recapitulate in reverse the developmental stages that once formed them? And when for instance hypoxic cells develop stem cell-like phenotypes, do they indeed become stem cells? These are central questions as they relate to the concept of tumor stem cells or tumor-initiating cells. Without formal proofs and experimental backup, we assume that the stem cell phenotype of tumor cells only mimics that of the cognate stem cell. However, as demonstrated in glioma, the glioma stem cells have the capacity of neural stem cells to give rise to distinct non-glial differentiation lineages (32,33). Thus, while the transcriptome of tumor stem cells A B Figure 2. Human neuroblastoma specimens stained for neuron-speciﬁc enolase by immunohistochemistry. Sections of a neuroblastoma bone- marrow metastasis (A) and a ganglioneuroma specimen (B), respectively, stained for neuron-speciﬁc enolase (ENO2) expression. Note that tumor cells differ considerably in neuron-speciﬁc enolase levels, both at a more immature (panel A) and at a differentiated (panel B) stage. Arrows show enolase-positive and arrow-heads show enolase-negative tumor cells.