Intervertebral disc (IVD) degeneration and associated low back pain (LBP) remains a major burden to our society without significant improvements in treatment strategies or patient's quality of life. While the recent cell-transplantation studies for treatment of degenerative disc disease have shown promising results, to better gauge the success and functional outcomes of these therapies, it is crucial to understand if transplanted cells give rise to healthy nucleus pulposus (NP) tissue. NP cell phenotype is unique and is defined by expression of a characteristic set of markers that reflect specialized physiology and function. This review summarizes phenotypic markers that mirror the unique physiology and function of NP cells and their progenitors and should be considered to when measuring outcomes of cell-based therapies to treat disc degeneration.