The goal of this clinical trial is to compare two strategies to monitor human cytomegalovirus (HCMV) infections in transplanted patients receiving letermovir (LTV) as anti-HCMV prophylaxis. HCMV infection after transplantation is diagnosed by detection of HCMV DNA in blood. However, due to the peculiar mechanism of action of LTV, most episodes of HCMV DNA detection are caused by release in the blood stream of non-infectious HCMV DNA. In true episodes of productive infection, HCMV DNA in blood is present inside the virion and therefore is resistant to DNAse digestion. Conversely, when non-infectious free-floating HCMV DNA is released in the bloodstream, it will be degraded after treatment of plasma with DNAse and will not be detectable by real-time PCR assays. Researchers will compare determination of HCMV DNA in blood with or without previous digestion of non-infectious free-floating DNA with DNAse. In patients of the Control group HCMV DNA will be tested without DNAse digestion. If HCMV DNA is positive, patients will stop LTV prophylaxis and receive antiviral therapy with another drug. In patients of the Study group HCMV DNA will be tested after DNAse digestion. Only if HCMV DNA is positive after DNAse digestion, patients will stop LTV prophylaxis and receive antiviral therapy with another drug. The main aim of the study is to demonstrate that, by avoiding inappropriate antiviral therapy during LTV prophylaxis, transplant patients will suffer of lower antiviral-drug-related toxicity. A monitoring strategy able to identify true episodes of HCMV productive infection during LTV prophylaxis will lead to a lower rate of inappropriate antiviral therapy and drug-related toxicity without an increased risk of HCMV disease.