Yeast artificial chromosomes (YACs) are genetically engineered chromosomes derived from the DNA of the yeast, Saccharomyces cerevisiae [https://www.genome.gov/genetics-glossary/Yeast-Artificial-Chromosome], which is then ligated into a bacterial plasmid.  By inserting large fragments of DNA, from 100–1000 kb, the inserted sequences can be cloned and physically mapped using a process called chromosome walking. This is the process that was initially used for the Human Genome Project, however due to stability issues, YACs were abandoned for the use of bacterial artificial chromosome [https://www.genome.gov/genetics-glossary/Bacterial-Artificial-Chromosome]
The bakers yeast S. cerevisiae' is one of the most important experimental organisms for studying eukaryotic molecular genetics.
Beginning with the initial research of the Rankin et al., Strul et al., and Hsaio et al., the inherently fragile chromosome was stabilized by discovering the necessary autonomously replicating sequence (ARS); a refined YAC utilizing this data was described in 1983 by Murray et al.
The primary components of a YAC are the ARS, centromere [https://www.nature.com/articles/nrm.2015.5], and telomeres [https://www.nature.com/articles/350569a0] from S. cerevisiae. Additionally, selectable marker genes, such as antibiotic resistance and a visible marker, are utilized to select transformed yeast cells. Without these sequences, the chromosome will not be stable during extracellular replication, and would not be distinguishable from colonies without the vector.