The aim of the present study was to quantify expression of TCII and TCII-R in a variety of naturally occurring, readily available canine and feline malignant tumors, to compare expression of these proteins between tumors and their corresponding adjacent normal tissues, and to determine whether tumor expression of TCII and TCII-R demonstrates a correlation with tumor proliferation status as assessed by measurement of Ki-67 expression. This evidence concerns the gene MKI67 and cancer.