Several tetraquark candidates have been reported by particle physics experiments in the 21st century. The quark contents of these states are almost all q q Q Q, where q represents a light (up, down or strange) quark, Q represents a heavy (charm or bottom) quark, and antiquarks are denoted with an overline. The existence and stability of tetraquark states with the qq Q Q (or q q QQ) have been discussed by theoretical physicists for a long time, however these are yet to be reported by experiments.