Replication asynchrony for the TIMP1 (T – white bars) and HPRT (H – grey bars) genes assessed by FISH. The bars show the frequency of nuclei exhibiting unreplicated (single/single); asynchronously replicated (single/double) and completely replicated (double/double) signals as shown in the schematic below. The TIMP1 and HPRT probes were individually hybridized to interphase nuclei of the same preparations. Two human female lymphoblast cell lines were examined for the approximate degree of replication asynchrony, one that inactivated TIMP1 (1 – GM07059) and one that expressed TIMP1 from the Xi (2 – HSC593). For TIMP1, 313 cells were counted from 3 separate cell harvests for cell line 1 (T1) and 217 cells for cell line 2 (T2) from two separate harvests. 144 cells were examined for HPRT from 2 slides of one harvest for cell line 1 (H1) and 97 cells from a single harvest for cell line 2 (H2). Between different cell harvests of the same cell line the maximal difference in frequencies of nuclei in each of the three replication classes was 3%.