Schematic of the FISH assays used by Hirsch et al¹  to assess chromosomal aneuploidy and allelic replication timing. Interphase FISH (bottom left) demonstrates the expected patterns for diploid and monosomy 7 cells using probes for chromosomes 8 (yellow) and 7 (magenta). The expected hybridization pattern for replication synchronous alleles using a single yellow probe will differ depending on where the cells are in cycle (bottom right). Postmitotic cells in G1 and early S phase (before replication) will exhibit a “singlet-singlet” signal corresponding to each allele, while cells that have undergone replication later in S phase will exhibit a “doublet-doublet” pattern corresponding to the 2 replicated alleles. Professional illustration by Marie Dauenheimer.