Third the sequencing output is directly detected without the need for electrophoresis; base interrogation is performed cyclically and in parallel. The enormous numbers of reads generated by NGS enabled the sequencing of entire genomes at an unprecedented speed. However, a drawback of NGS technologies was their relatively short reads. This made genome assembly more difficult and required the development of novel alignment algorithms (see below). The first NGS technology to be released in 2005 was the pyrosequencing method by 454 Life Sciences (now Roche) [5]. The 454 Genome Sequencer generated about 200 000 reads (20 Mb) of 110 base-pairs (bp). One year later, the Solexa/Illumina sequencing platform was commercialized (Illumina acquired Solexa in 2007). The third technology to be released was Sequencing by Oligo Ligation Detection (SOLiD) by Applied Biosystems (now Life Technologies) in 2007 [6]. The Illumina and SOLiD sequencers generated much larger numbers of reads than 454 (30 and 100 million reads, respectively) but the reads produced were only 35 bp long. In 2010, Ion Torrent (now Life Technologies) released the Peronal Genome Machine (PGM). This system was developed by Jonathan Rothberg, the founder of 454, and resembles the 454 system. An important difference is that the PGM uses semiconductor technology and does not rely on the optical detection of incorporated nucleotides using fluorescence and camera scanning. This resulted in higher speed, lower cost, and smaller instrument size. The first PGM generated up to 270 Mb of sequence with up to 100 nt reads; slightly shorter than those produced by 454. For a detailed description of these major NGS technologies, see [7,8]; a quick overview is presented in Box 1.