C/EBPα and C/EBPϵ up-regulate Per2 expression in human leukemia cell lines. (A) KCL22, Daudi, and U937 cells stably transfected with zinc-inducible C/EBPα (pMTα), C/EBPϵ (pMTϵ), or control empty vector (pMT) were incubated in the presence of zinc for 24 hours. Total RNA was extracted and analyzed by real-time PCR with Per2- and Rev-Erbα-specific primers. (B) Per2 expression levels were measured by real-time PCR in samples from 4 normal human lymph nodes and the indicated lymphoma cell lines. The results are expressed in arbitrary units as a ratio of either Per2 (A-B) or Rev-Erbα (A) transcripts to 18S transcripts (each value represents the mean of 3 measurements of the sample).