Table II. Mitotic stability depends on the timing of Chl4p loss relative to introduction of centromere DNA.

Plasmid	Wild type		chl4Δ→plasmid		Wt + plasmid→ chl4Δ
	Stable(80–90%)	Unstable	Stable	Unstable(0.08–5%)	Stable(60–90%)	Unstable(0.08–5%)
pYe (CEN3) B	10/10	0/10	0/15	15/15	12/27	15/27
pYe (CEN3) 30	10/10	0/10	0/15	15/15	20/36	16/36
pYe (CEN3) 41	10/10	0/10	0/15	15/15	17/32	15/32

Centromere plasmids (pYe [CEN3] B, pYe [CEN3] 41, or pYe [CEN3] 30) were introduced into wild type and chl4Δ mutants (left and middle columns). Plasmid-bearing cells were selected, and plasmid stability was determined as described in the Materials and methods. Centromere plasmids are stably segregated (80–90%) in 30/30 wild-type cells (left column). In 45/45 chl4Δ mutants, <5% of cells contained plasmids upon nonselective growth. In the right column, cells containing the indicated centromere plasmids were transformed with chl4Δ::KAN^r. Mitotic stability measurements revealed heterogeneity in plasmid stability: 60–90% stability (49/95) and 0.08–5.0% stability (right column, 46/95).