Noncanonical NF-κB is involved in Tax-mediated cellular transformation and tumorigenesis. (A) Kaplan-Meier plot of tumor-free mice. Tax⁺ p100^+/+ mice (n = 17), Tax⁺ p100^−/− mice (n = 20), and Tax-free p100^+/+ or p100^−/− mice (n = 9) were monitored daily for tumor formation for up to 52 weeks. A representative mouse with tumors is shown (right) and the tumors are indicated by arrows. (B) Protein expression levels of NF-κB proteins in Tax⁺p100^+/+ or Tax⁺p100^−/− tumor cells. (C) Involvement of p52 for Tax-mediated colony formation. The p100^−/− MEFs stably expressing Tax alone or together with p100, p52, or p100SS/AA were plated in soft agar for colony formation. The colony formation efficiency is represented as a percentile of that in p100^+/+ MEFs stably expressing Tax (arbitrarily set as 100). These data represent mean (± SD) for n = 3 samples. **P < .01. (D) Protein expression levels of Tax, p100, p52, and p100SS/AA in the MEFs stably expressing Tax used in panel C. (E) Suppression of Tax-mediated colony formation by IκBαSS/AA. The p100^+/+ MEFs stably expressing Tax alone or together with HA-IκBαSS/AA were plated in soft agar and subjected to colony formation as described in panel C. (F) Protein expression levels of Tax and HA-IκBαSS/AA in the MEFs used in panel E.