ID1 and neoplasm: Furthermore, the reduction of Id1 expression impaired the in vivo tumor-initiating ability of orthotopic xenografts (n = 5; frequency: 1/36,920 cells compared to 1/1,090 cells, P < 0.0001, Poisson distribution; Figure 4E, right panel), as well as in vivo tumor growth (Figure 4E, left and middle panels).