**4. The Human Genome as a Universal Reference Standard**

Seventeen years have passed since the joint announcement of the human genome sequence [14,15]. This period has seen a number of attempts to complete the assembly, applying a variety of technologies [16,17]. All of these have reported a smaller genome size than what has, as of the end of 2017, been suggested in GRCh38.p12, the most recently released Genome Reference Consortium version, which comprises 3,257,319,537 bp. Assuming the Doležel et al. [18] conversion of 1 pg = 0.978 Gbp, 3.5 pg 1C DNA is equivalent to 3,423,000,000 bases. Thus, the 7 pg value represents an ~5.1% over-estimate of the GRCh38.p12 assembly prediction. This difference lies at the lower end of the error range predicted by Doležel and Greilhuber [13]. Given that the human reference genome is still incomplete, the expectation is that the gap between the 7 pg figure and the "real" human genome size will continue to diminish. Nevertheless, a 5% error is not dissimilar to the variation observed between estimates of nuclear DNA amounts of a given species produced by different laboratories [19,20]. Thus, the recommendation remains that the 7 pg figure continue to be used as the reference for measuring 2C values of both animal and plant genomes.
