RECQL4 and Rothmund-Thomson syndrome: Difficulties in establishing whether this cellular phenotype might be a useful adjunct to clinical diagnosis are due to: i) the description of chromosomal instability in RTS patients before the RECQL4 molecular test became available, making uncertain in these cases the relationship of the observed phenomenon with RTS caused by the RECQL4 defect; ii) the variability in the applied procedure as either spontaneous or induced chromosomal instability was monitored and iii) the variability in the cells used for testing, which include lymphocytes, fibroblasts and lymphoblastoid cell lines.