Hi. There are five USART peripherals in STM32L151xD, STM32L152xD devices. But when i chose STM32L151VD device in target options, i can see only 3 USARTs (USART1, USART2, USART3) in system viewer (when debugging) and UART4, UART5 checkboxes are not present in RCC window too. However, this peripherals present in STM32L1xx std peripherals library, and works well.