PCNA and infection: Mutation of the p21 PIP box (p21ΔPCNA, Figure 4A) abrogated interaction with PCNA (Figure 4D, compare lane 3 to 2), and the p21ΔPCNA protein expressed in murine cells was not degraded as efficiently as the wild-type protein (p21WT) following infection (Figure 4E, compare lanes 8 and 6 to 4 and 2).