A base station may cancel or delay (e.g., pre-empt) a transmission from a wireless device by sending a pre-emption indication to the wireless device. Based on receiving the pre-emption indication, the wireless device may cancel the transmission, store an indication associated with the cancelled transmission, and/or store the cancelled transmission in a data buffer. The base station may request transmission of the data unit (e.g., at a later time) by transmitting another indication.