To assess the tumorigenicity of D4M melanoma cells, we intradermally injected a range (300–300 000) of cells into B6 and NSG host mice. Each concentration of cells was injected into four B6 and four NSG mice. Of the eight mice injected with 300 cells, no tumors developed in any of the mice after 27 weeks. In contrast, it took only two weeks for tumors to be observed in mice that received ≥3000 cells, and only one week for tumors to be observed in mice that received 300 000 cells (Table 1). Tumors arose with similar kinetics in B6 and NSG host mice; there was no significant difference in the average tumor size between NSG and B6 mice injected with 3000