POP1 and systemic sclerosis: In SSc, the most common autoantigens that anti-Th/To are directed towards are homologs of processing of precursor 1 (hPOP-1), ribonuclease P/MRP subunit p25 (RPP25), ribonuclease P/MRP subunit p40 (RPP40) and ribonuclease P/MRP subunit p30 (RPP30) [32].