Expression of ICSBP in ICSBP^-/- cells activates the Ink4b promoter. Tot2 cells were cultivated for 2, 3, 4, or 6 days following transduction with an ICSBP-expressing vector. The cells were harvested and total RNA was isolated. The p15^Ink4b cDNA was amplified by RT-PCR using 50 ng of total RNA. To control for sample loading, β-actin cDNA was amplified from the same RNA samples.