A new polyomavirus, Merkel cell polyomavirus, was identified from Merkel cell carcinoma, a rare skin cancer. Origin of Merkel cell carcinoma is Merkel cell, a neuroendocrine cell in the skin. Merkel cell carcinoma occurs in the skin of head and face of white elders. Like other polyomaviruses, a 5.4 kbp-virus genome encodes VP1, VP2, VP3, small T antigen, and large T (LT) antigen. MCV has been frequently detected in Merkel cell carcinoma in the world by PCR. It was demonstrated that MCV genome integrated into the host genome of Merkel cell carcinoma. LT plays an important role in replication of a circular MCV. However, mutations with stop codons were identified in the LT genes derived from Merkel cell carcinoma cases. Such mutations cause a truncation of the LT gene, resulting in defect of LTs helicase activity and in induction of Rb binding function in the LT. MCV is a new member of human oncovirus belonging to the human polyomavirus.