ABSTRACT: Chondrocytes advance through the long bone growth plate in synchrony, allowing their segregation into one of four zones each corresponding to a different phase of maturation. Beginning from the epiphysis the recognized zones are: reserve, proliferative, prehypertrophic, and hypertrophic. A fifth zone, the perichondrium is located in apposition to the growth plate at its circumferential surface. The progression of chondrocytes through these zones, and hence growth, is regulated by a complex interplay of signaling pathways, defects in which lead to osteochondrodysplasias. At present, there are over 370 recognized skeletal disorders, slightly more than half of which have been explained at the molecular level. Through a combination of laser microdissection, linear mRNA amplification, and GeneChip analysis, our laboratory has acquired the zone specific expression profile of chondrocytes from the distal femoral growth plates of two normal patients. On average, transcripts interrogated by 12,193 or 18,454 GeneChip probe sets (out of a possible 54,000 probe sets) were present in patients 1 and 2, respectively. The homologous inter-array correlations for the log(2) intensities were M-bM-^IM-% 0.91M-BM-10.01. Keywords: Chondrocyte differentiation program. Populations of cells corresponding to the hypertrophic, prehypertrophic, proliferative, reserve, and perichondrium, were separately isolated by laser microdissection from cryostat sections of human growth plates using a Leica AS-LMD microscope. RNA was prepared from the isolated cells, amplified using a T7 linear amplification protocol, and analyzed by hybridization to Affymetrix microarrays. The distal femoral growth plate (left or right) was exclusively used for this study. Patient 1 of this study is an 11-10/12 year old Caucasian female and patient 2 is a 13-3/12 year old Caucasian male.