Quorum-sensing (QS) involved in the production of N-Acylhomoserine lactones (AHLs) is a universal way of communication of gram-negative bacteria. Complete AHL-QS system includes pairs of AHLs synthase belonging to LuxI family and cognate LuxR-family AHLs sensor-regulator. However, many gram-negative bacteria have evidenced the presence of AHL-QS related LuxR-type genes, which are unpaired to a cognate LuxI. These unpaired LuxRs have been called solos or orphans. LuxR solos are thought to be important for bacterial signal perception in eavesdropping, intra-species and inter-kingdom communication, which become research topic in the field of QS. Here, the finding, concept, protein structure, and main types of LuxR solos are illustrated. Furthermore, the function and important protein of LuxR solos associated with sensing AHLs or non-AHLs are reviewed. The prospect and significance of quorum sensing LuxR solos in bacteria are also discussed.