Repetitive nature of the ribosomal DNA (rDNA) gene makes the sequencing of hundreds copies of mammalian 45S rDNA (about 45-kb per copy) extremely difficult and its assembly is often excluded. Increasing evidence shows that 45S rDNA variations (copy number or single nucleotide), structural ribosomal RNA (rRNA) transcript variants, and non-structural rRNA transcripts (sense and anti-sense long noncoding rRNAs that include promoter rRNAs, and rRNA-derived fragments) play essential roles in mammalian development and diseases. Complete pictures of the hundreds copies of 45S rDNA and their rRNA transcripts require further innovation in sequencing techniques that include bioinformatics. The advancements in mammalian rDNA and rRNA sequencings and the discoveries of novel functions of the rDNA variants and rRNA transcripts are discussed here.