ISG15 mRNA expression levels in macrophages and B lymphocytes. Semiquantitative RT-PCR analysis showing the mRNA expression levels of the ISG15 gene in primary macrophages (A) and B lymphocytes (B) from wild-type and ICSBP-deficient mice. The PCR protocols were standardized for both ISG15 and the internal control β-actin to ensure that PCR amplification was in the linear phase.