Over-expression of the multifunctional zinc-finger transcription factor Yin Yang 1 (YY1) has been associated with cellular proliferation and resistance to apoptotic stimuli. In this study, we report that YY1 was uniformly highly over-expressed in a wide range of human cancer cell lines and in human colon cancer tissue samples. The examination of YY1-specific mRNA expression demonstrated at least six mRNA isoforms ubiquitously expressed in normal human adult and fetal tissues. Substantial over-expression of two specific mRNA isoforms of 7.5 and 2.9 kb size, respectively, was detected in gastrointestinal and other cancer cells in vitro, whereby mRNA stability differed significantly between various cell lines. YY1 protein expression levels were similar in different colon cancer cell lines. Using FISH analysis of several colorectal cancer cell lines, the human YY1 locus was expectedly identified on chromosome 14q32 and no evidence of gene amplification and chromosomal translocation was observed. However, varying degree of aneuploidy was noted in vitro. YY1 immunoreactivity in human colon tumor samples was found more intense in poorly differentiated tumors than in moderately and well differentiated colon cancers and lower expression levels tended to be associated with shorter survival. In conclusion, YY1 was over-expressed in colon cancer in the absence of gene amplification and chromosomal translocation. YY1 mRNA and protein stability are important regulatory mechanisms of YY1 expression in colon cancer.