a, Analysis of HP1a 206 amino acid protein sequence. Top, known domains chromo, hinge, and shadow. Black line, predictor of natural disordered regions (PONDR) score for intrinsic disorder, >0.5 is considered disordered. Red line, hydropathy score (positive is hydrophobic). Yellow bars indicate low-complexity sequences. b, 1 mg ml−1 HP1a in 50 mM NaCl was incubated at 37 °C for 5 min, then returned to room temperature (22 °C) and imaged with differential interference contrast microscopy (DIC) every 5 s for 8 min. Quantification of average number and area of HP1a droplets formed in a 50 × 50 μm window, n = 3. c, Probability distribution of droplet aspect ratio. d, Probability distribution of droplet area on a log–log plot follows a power law exponential with τ = −1.5, characteristic of aggregating systems. e, f, Sediment gradient analysis shows large oligomers of HP1a in 0.05 M NaCl (arrows, e) but not 0.5 M NaCl (f). g, Two-colour images showing HP1a and H2Av for one nucleus over Drosophila embryonic cycle 13. h, i, Quantification of average HP1a domain size per nucleus (sum of all foci in each nucleus) (h) and total intensity of fluorescently-tagged HP1a and H2A (i), over embryonic cycles 10–14. Error bars are s.d. n = 12 embryos of >75 nuclei each. j, Two-colour images of HP1a and H2A showing differentially shaped heterochromatin domains in Drosophila embryos, adult gut and cultured Kc cells, and mouse fibroblast NIH3T3 cells.