Section 4.1 Radiation-induced chromosome aberrations I would like to draw attention to the importance of data derived from the study of occupational exposure when discussing low dose effects. Support for a linear dose relationship extending to very low doses has come from the study of translocation frequencies in workers at the Sellafield nuclear facility. A linear dose response relationship is found with cumulative dose. However, it should be remembered that these cumulative doses over several hundred mSvs were accumulated as extremely small daily increments of < 0.4mSv and although the effects of such low doses are immeasurable their cumulative effect conforms to an additive model giving a linear dose response which conforms to expectations dereived from the linear component of in vitro dose response curves. This work is discussed in more detail in: E. J. Tawn, C.A. Whitehouse R.E. Tarone R E (2004) FISH chromosome aberration analysis on retired radiation workers from the Sellafield nuclear facility. Radiat. Res. 162 249-256. Section 4.3.2 Radiation induced genomic instability I would like to question the sentance " Evidence for transmissible instability in irradiated human populations is inconsistent (Nakanishi et al 2001; Whitehouse and Tawn, 2001)" I think it would be more appropriate to say that it had not been adequately demonstrated. The publication by Nakanishi has been critically reviewed by Little and this is acknowledged in the references. Two papers from our group have failed to find evidence of the induction of persistent genomic instability. That on plutonium workers is referenced but an earlier paper, E.J. Tawn, C.A. Whitehouse, F.A. Martin (2000) Sequential chromosome aberration analysis following radiotherapy - no evidence for enhanced genomic instability, Mutat. Res. 465 45-51, also reports no evidence for the induction of persistent instability. Further support for this absence of an effect of induction and transmission in humans in vivo can be found in L.G. Littlefield, L.B. Travis, A.M. Sayer, G.L. Voelz, R.H. Jensen, J.D. Boice, Jr., Cumulative genetic damage in hematopoietic stem cells in a patient with a 40-year exposure to alpha particles emitted by thorium dioxide, Radiat. Res. 148 (1997) 135-144.. and S. Salomaa, K. Holmberg, C. Lindholm, R. Mustonen, M. Tekkel, T. Veidebaum, B. Lambert, Chromosomal instability in in vivo radiation exposed subjects, Int. J. Radiat. Biol. 74 (1998) 771-779.