PRG4 and bone disorder: In conclusion, we have produced a set of sensitive mAbs that are specific to a repetitive, glycosylated, octapeptide motif of human lubricin, and used one of them to improve the laboratory diagnosis of a Mendelian genetic skeletal disease CACP, to distinguish degradation of lubricin that occurs intracellularly from that which occurs extracellularly, and to begin delineating how monitoring the extracellular degradation of lubricin can be used to assess joint health.