SLCO2A1 and primary hypertrophic osteoarthropathy: Genetically, PHO is classified into two primary autosomal recessive subtypes: (1) PHO autosomal recessive 1 (PHOAR1) caused by HPGD gene mutations [5]; (2) PHO autosomal recessive 2 (PHOAR2) caused by SLCO2A1 gene mutations [11].