According to the molecular findings, PHO has been categorized into two subtypes: (1) hypertrophic osteoarthropathy, primary, autosomal recessive, type 1 (PHOAR1; MIM 259100), caused by HPGD deficiency and (2) hypertrophic osteoarthropathy, primary, autosomal recessive, type 2 (PHOAR2; MIM 614441), caused by SLCO2A1 deficiency. The gene discussed is HPGD; the disease is primary hypertrophic osteoarthropathy.