We started to determine the mechanism of hyaluronan accumulation in ovarian cancer by analyzing the expression profiles of hyaluronan synthases and hyaluronidases in a clinically defined set of tumors, and found that a significantly decreased HYAL1 expression correlates with lower hyaluronidase activity and elevated hyaluronan content of the tumors, while HAS expression was not as consistently associated to the accumulation of hyaluronan. The gene discussed is HYAL1; the disease is ovarian cancer.