Cells' ability to efficiently replicate their genomes is essential for regulating chromosomal division and maintaining chromosome integrity.  Defects in any of these cellular processes may cause genomic instability, potentially leading to cancer.  The Chaos3 allele in the yeast Saccharomyces cerevisiae is a single base pair change causing an amino acid substitution in the Mcm4 protein.  Mcm4, a component of the replicative helicase, is recruited to replication origins to unwind double stranded DNA and initiate replication.  Chaos3 is in a region of MCM4 that is highly conserved across eukaryotes; while mutations in conserved regions are generally non-viable, Chaos3 is a viable allele that causes genomic instability, leading to elevated cancer rates in mice.  In S. cerevisiae, Chaos3 decreases early firing of the autonomously replicating sequences (ARS) where DNA replication begins.  Chaos3 does not affect all early firing ARSs in the genome; rather, a large proportion of origins near centromeres, thereby delaying replication of those centromeres.  Essential for chromosome segregation, the centromere is the location where spindle fibers attach to pull apart sister chromatids during cell division.  I hypothesize that this delay in centromere replication results in chromosomal instability, including the loss of a chromosome.  I am using CRISPR guided cutting directed by a customizable guide RNA to replace centromeric adjacent ARS510, that has decreased firing levels in Chaos3, with unaffected, early firing ARS305, to see if firing levels in the mutants are affected based on ARS chromosome location (i.e., proximity to a centromere) or ARS sequence.  If replacing ARS510 with ARS305 restores early origin firing in this region this will confirm the Chaos3 mutation affects specific ARS sequences rather than ARS location on the chromosome.  Furthermore, if centromere replication delays are the cause of genomic instability in Chaos3, this ARS replacement should rescue chromosome loss.