Thus, the dynamically phosphorylated CTD temporally couples transcription with other nuclear processes by NLG919 serving as a scaffold for recruitment of various proteins involved in transcription, chromatin modification, and RNA processing. Therefore, regulation of CTD phosphorylation patterns during the transcription cycle by CTD kinases and phosphatases is crucial for proper gene expression. Ssu72 is a well-studied CTD phosphatase in yeast. The Ssu72 gene was originally identified in budding yeast as an essential gene that genetically and physically interacts with the general transcription factor TFIIB and affects the precision of transcription start site selection. Subsequently, Ssu72 was shown to be a subunit of cleavage and polyadenylation factor holo-complex that is involved in 39-end processing of some pre-mRNAs and in transcription termination of small noncoding RNAs such as snoRNAs, snRNAs, and cryptic unstable transcripts. Although yeast Ssu72 was initially shown to be a Ser5P-specific CTD phosphatase, recent studies have demonstrated that it also exhibits Ser7P phosphatase activity in vitro. Consistent with this, loss of Ssu72 in vivo results in an increase in the phosphorylation level of both Ser5 and Ser7, both at snoRNA genes and in the 39 regions of mRNA genes. The mammalian ortholog of yeast Ssu72 was originally identified as a binding partner of the tumor suppressor RB. Although mammalian Ssu72 is very similar at the sequence level to yeast Ssu72 and can also associate with TFIIB, it is unable to rescue a lethal ssu72 mutation in yeast, and its suppression does not affect cell proliferation or viability of mammalian Ethambutol hydrochloride cultured cells. Thus, mammalian Ssu72 may share a subset of the functions of the yeast protein but also exert specific functions in mammalian cells. Although recent studies demonstrated that human Ssu72, like its yeast counterpart, exhibits Ser5P and Ser7P-specific CTD phosphatase activity in vitro, its in vivo roles in CTD dephosphorylation and gene expression remain to be elucidated. To investigate the functions of vertebrate Ssu72 at the cellular level, we developed chicken DT40 B-cell lines in which Ssu72 expression is conditionally inactivated. Ssu72 depletion caused defects in 39-end formation of U2 and U4 snRNAs and GAPDH mRNA.