CD24 and intervertebral disk degenerative disorder: Using these markers, NP cells were classified into four subtypes: dormant stem cells (Tie2+GD2−CD24−), self‐renewing stem cells (Tie2+GD2+CD24−), committed NP progenitor cells (Tie2−GD2+CD24+), and mature NP cells (Tie2−GD2−CD24+).117 Identification of these cell surface markers is epoch‐making in that it enables to evaluate the severity of IDD by quantifying the cell number and function, and in that it makes an index of induction of differentiation from other sources to become NP progenitor cells.