A transmitting system and a method of transmitting digital broadcast signal are disclosed. The method of transmitting digital broadcasting signal in a transmitter includes multiplexing a specified number of mobile data packets, a first scalable number of mobile data packets, and a second scalable number of main data packets, interleaving mobile data in the mobile data packets and main data in the main data packets, transmitting the interleaved mobile and main data during a slot, wherein the data group includes a plurality of first blocks, each first block including a plurality of data segments, specified block of the plurality of first blocks including the mobile data, known data sequence, trellis initialization data and RS parity bytes inserted in pre-determined position of the data group, wherein the data group includes a plurality of regions, wherein the last region includes the first scalable number of mobile data packets.