Although chromatin condensation is a well-known hallmark of apoptosis, the generation mechanism has not been clarified. Histone H1, a positively-charged abundant nuclear protein, is located in the linker region of chromatin. There are several Histone H1 subtypes that are encoded by variant genes. Using serial histone H1-deletion mutant cells established from the chicken B-cell leukemia line DT40, we found that apoptotic chromatin condensation was decreased in relation to histone H1 protein level and that the chromatin in nuclei prepared from the live null mutant cells had a high accessibility of DNases and transposase. This indicated that linker histone H1 was the general chromatin condensation factor and that the loss of histone H1 generated open chromatin in both apoptotic and live cells. (C) 2019 Elsevier Inc. All rights reserved.