For decades, reference genomes carried a hidden asterisk. Even the best assemblies contained gaps, particularly in regions packed with repetitive sequences, where short reads pile up but fail to resolve the underlying order. Centromeres, telomeres, and the short arms of certain human chromosomes were left blank or approximated, leaving a significant portion of every genome functionally dark.
But this changed in 2022 with the teleomere-to-telomere (T2T) consortium’s landmark publication of the complete human genome, filling these final gaps. Catalyzed by this breakthrough, Cell and Cell Genomics has now recently published a special collection of diverse T2T genome papers, representing some of the most complete chromosomal assemblies ever produced for any species. Spanning humans, primates, songbirds, horses, rats, voles, and more, the collection draws on PacBio HiFi sequencing as a central thread connecting the studies.
As T2T Consortium co-founders Adam Phillippy, Karen Miga, and colleagues wrote in an accompanying Cell commentary, “We finally have the ability to read the complete genome of any human and (nearly) any species.” What makes this collection particularly striking is not just the assemblies themselves, but how the methods and workflows are evolving alongside them, with T2T moving from a landmark technical achievement toward something that can be applied across the tree of life.
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How long-read sequencing makes complete genome assembly possible
A true T2T genome assembly requires a genome to be reconstructed from one chromosome end to the other, with no sequence missing in between. Reaching that standard requires reads long enough to span complex repetitive elements that trip up shorter technologies. PacBio long-read sequencing, which produces reads averaging 10 to 25 kilobases with accuracy exceeding 99.9%, has become the foundational data source for this work. Where a shorter read would struggle in a repetitive region and leave a gap, a HiFi read is long enough to span the whole region with a single, mappable stretch of sequence.
A ground truth for the whole genome
One of the centerpiece papers updates the benchmark for the human genome itself. Hansen et al. present a complete, near-perfect diploid assembly of the well-characterized HG002 reference sample, adding roughly 700 megabases of autosomal sequence and over 200 megabases of sex chromosome sequence that were absent from prior Genome in a Bottle benchmarks. This assembly provides a true ground truth for the entire genome rather than just the regions short reads can access, and near-perfect assembly accuracy translated to an order-of-magnitude improvement in variant calling performance. A companion paper by Lin et al. extends this analysis, assembling 156 phased copies of the short arms of acrocentric chromosomes across a four-generation pedigree. Their work reveals that these heavily repetitive regions mutate at ten times the background rate seen elsewhere in the genome, with a mutational signature pointing to distinct DNA repair mechanisms.
Complete genome assemblies across species with HiFi sequencing
Notably, this collection extends well beyond human genomics. Formenti et al. present a phased diploid T2T reference for the zebra finch, a key model for vocal learning research, recovering 90 megabases of previously missing sequence and 2,710 genes absent from prior assemblies. In non-human primates, Hebbar et al. assembled a complete marmoset genome including 500-plus newly identified lineage-specific transcribed genes, while Zhang, Xu et al. resolved the rhesus macaque reference and defined four distinct satellite architectures in its subtelomeric regions that had previously eluded characterization. Li, Cappellitti et al. produced T2T assemblies for both horse and donkey from a single mule sample, with both now adopted as NCBI reference genomes. This work reveals the species’ unusual centromere organization, including satellite-free centromeres, that distinguishes equids from other mammals. Also by Li et al., a separate paper assembles the first T2T rat genome from solid tissue, producing 16 of 22 chromosomes without any gaps and revealing novel meiotic interactions between the sex chromosomes.
Single-library HiFi and CiFi sequencing for diploid genome assembly
One of the more significant developments in the collection is how data generation itself is becoming more streamlined. Abuelanin et al. generated near-T2T diploid genomes for prairie and meadow voles prepared from a single HiFi run, with the CiFi method providing the chromatin interaction data needed for chromosome-scale phasing alongside the assembly. This kind of single-library, single-run approach represents a meaningful step toward making diploid T2T assembly more accessible and efficient. This study also resolved a prairie vole-specific duplication in a vasopressin receptor gene that is both implicated in pair bonding behavior and studied in the context of autism research, an example of how complete assemblies can unlock biological questions that were previously out of reach.
Bioinformatic tools for telomere-to-telomere genome finishing
Publishing complete genomes at scale also demands better computational tools. Kim et al. introduce Verkko-Fillet, a Python framework for interactively inspecting and editing assembly graphs, bridging the gap between automated graph-based assembly and the manual curation often needed to finish complex regions. In a complementary approach, Antipov et al. present TTT (trivial tangle traverser), an algorithm that automates gap closure in the most difficult assembly regions, achieving 91% agreement with expert manual curation on the HG002 assembly. Together, these tools help close the gap between a high-quality draft and a truly finished genome.
What T2T genome assembly makes possible
Taken together, these studies illustrate how the field is shifting. T2T assemblies are no longer a proof of concept reserved for model organisms with dedicated teams; they are becoming precise enough to serve as reference standards, broad enough to span dozens of species, and efficient enough to emerge from streamlined single-library workflows. The sequences these assemblies finally capture are not just technical milestones; they include regions tied to disease, development, and gene regulation that have been functionally inaccessible until now. With HiFi sequencing providing the read length and accuracy that make finishing possible across a growing range of organisms and questions, that hidden asterisk can finally be removed.
If these studies have you thinking about what’s possible for your own research, we can help you find out. Get a personalized HiFi Fit Kit tailored to your species, goals, and questions.