The iconic double helix structure of DNA was deduced in the classic model of Francis Crick and James Watson in 1953. Seminal work by Crick, Sydney Brenner, Marshall Nirenberg and others helped crack the genetic code in the 1960s, revealing the triplet codon schema that corresponds to each of the 20 amino-acid building blocks of proteins. The development of DNA sequencing techniques in the late 1970s revealed the sequence of genes, as well as the dramatic revelation that eukaryotic genes are punctuated by non-coding introns. The portion of the human genome that encodes genes is surprisingly low, although a decade after the Human Genome Project, the ENCODE Project showed that as much as 80 percent of the genome has some biochemical or functional activity.