We have generated tumorigenic (S2N) and non-tumorigenic (S2), normal-like to basal-like breast cancer cell lines from primary tumors. At high in vivo inoculation cell numbers of 10^6 cells/mouse both S2N and S2 monolayer as well as sphere culture cells grew at similar rates. However, at low inoculation cell numbers down to 10^3 cells only S2N sphere cells generated xenograft tumors. mRNA profiling revealed a unique cluster pattern of the tumorigenic S2N sphere cells, but a detailed analysis of TIC relevant transcription factors like Oct3, Sox and Nanog family members, Myc, Slug or Twist1 revealed no consistently increased expression in the highly tumorigenic cell lines. Our data indicate that the intrinsic genetic and functional markers investigated are not solely indicative of the in vivo tumorigenicity of putative breast tumor-initiating cells.