Comparison of the frequency of occurrence of restriction fragment length polymorphisms in control human DNAs and DNAs from infants dying from sudden infant death syndrome has indicated no significant difference in the case of restriction fragment length polymorphisms associated with the heat shock protein genes hsp7O and hsp9O. A highly significant difference was detected, however, in the case of the specific restriction fragment length polymorphisms detected by an hsp6O gene probe in MspI digests. (Arch Dis Child 1996;75:451-452) Keywords: heat shock proteins, restriction fragment length polymorphisms, sudden infant death syndrome, heat shock protein 60.Heat shock proteins play a central part in the normal physiology of cells.' Although they appear to be involved in the acquisition and maintenance of thermotolerance, other major roles include the repair of denatured cell proteins and functions as molecular 'chaperones' in the intracellular transmembrane transport of cellular proteins and in the construction of complex quaternary cellular protein structures. At present the cause of sudden infant death syndrome (SIDS) is unclear. Some deaths have been attributed to fever-like disorders,2 possibly brought on by overwrapping or by a virus infection. A possibility is that infants are susceptible to SIDS because some of the heat shock proteins are not produced or are defective, thus providing inadequate thermal protection. This could arise from defects in the DNA associated with genes encoding critical heat shock proteins. Results and discussion An initial screen of healthy human subjects was undertaken to determine whether sequence probes to the heat shock protein genes, hsp6O, hsp7O, and hsp9O could reveal any restriction fragment length polymorphisms (RFLPs). A panel of 26 genomic DNA samples was digested with 19 different restriction enzymes to detect RFLPs. Use of the hsp7o probe detected variations in these control samples digested with BgII, PstI and PvuII, as previously reported by Goate et al. 4 The DNAs digested with MspI or ScaI were also polymorphic for hsp6O, and the hsp90 probe detected variations in samples digested with Sau961. The frequency of the variable fragments was determined for all polymorphisms (table 1). The RFLPs detected using the hsp60 probe were more complex than those found using the hsp7O probe. With MspI, 16 constant fragments and six variant fragments were detected in 26 individuals. There was one hsp9o associated polymorphism, detected with Sau961 digested DNA. Nine of the fragments were invariant, whereas two others displayed a polymorphism characteristic of a two allele system.To determine if there was any difference in the nature of heat shock protein gene polymorphisms in infants dying from SIDS compared with the controls, 12 DNA samples from SIDS infants were examined with the same 19 restriction enzymes used in the initial screening for heat shock protein polymorphisms. Although the sample size of SIDS DNAs used was small it was sufficiently large to detect frequent...