The fact that we did not observe any significant differences in total cell numbers (between control and OM-CM cultures) could be explained by the following assumptions: 1) mitogens are present at a too low concentration to readily affect progenitor proliferation, 2) their activity is dependent of the status of the proteins (e.g. monomeric or dimeric form, oxidized) and therefore mitogens present in the OM-CM may be inactive, or 3) olfactory bulb and olfactory epithelial neural progenitor cells may require distinct factors cooperating with bFGF and/or EGF to stimulate their proliferation. Here, EGF is linked to ocular melanoma.