This study investigates a group of bacteria comprising more than 15% of the domain Bacteria, known as the candidate phyla radiation (CPR). The authors reconstructed complete and draft genomes from bacteria representing over 35 phyla, documenting unique features that distinguish these organisms from other bacteria. These organisms are characterized by small genomes, lack of numerous biosynthetic pathways, and the presence of self-splicing introns and proteins encoded within their 16S rRNA genes. They also exhibit unusual ribosome compositions, with many lacking specific ribosomal proteins and biogenesis factors. The study found that 50–100% of organisms sampled from specific phyla would evade detection in typical cultivation-independent surveys due to divergent 16S ribosomal RNA (rRNA) gene sequences. The authors propose that this group, which may comprise over 15% of the bacterial domain, has a shared evolutionary history and describe it as the candidate phyla radiation (CPR).This study investigates a group of bacteria comprising more than 15% of the domain Bacteria, known as the candidate phyla radiation (CPR). The authors reconstructed complete and draft genomes from bacteria representing over 35 phyla, documenting unique features that distinguish these organisms from other bacteria. These organisms are characterized by small genomes, lack of numerous biosynthetic pathways, and the presence of self-splicing introns and proteins encoded within their 16S rRNA genes. They also exhibit unusual ribosome compositions, with many lacking specific ribosomal proteins and biogenesis factors. The study found that 50–100% of organisms sampled from specific phyla would evade detection in typical cultivation-independent surveys due to divergent 16S ribosomal RNA (rRNA) gene sequences. The authors propose that this group, which may comprise over 15% of the bacterial domain, has a shared evolutionary history and describe it as the candidate phyla radiation (CPR).