POP4 is a protein subunit of ribonuclease P (RNase P), a ribonucleoprotein complex essential for tRNA maturation through 5'-end processing 1. As a core component, POP4 is uniquely conserved across all eukaryotes and archaea, making it the only RNase P protein subunit identifiable in all organisms with available genome sequences 1. The protein functions within the nucleolus and nucleoplasm as part of both RNase P and MRP (mitochondrial RNA processing) complexes, participating in tRNA 5'-leader removal and rRNA processing. Beyond its enzymatic role, POP4 has emerged as a stable reference gene for quantitative PCR studies across various cell types and pathological conditions 2345. In disease contexts, POP4 shows clinical relevance as it is significantly overexpressed when amplified in the 19q12 amplicon found in ER-negative grade III breast cancers, where its expression is required for cancer cell survival 6. The gene's stable expression patterns and functional importance in RNA processing make it both a reliable experimental control and a potential therapeutic target in specific cancer subtypes characterized by genomic amplification of its chr19 region.