Mechanisms of radiation carcinogenesis are being studied in two different systems (i) radiation-transformed retina pigment epithelial cells (RPE) and (ii) primary cultures of human childhood papillary thyroid carcinomas from Belarus. Metaphase spreads are investigated with cytogenetic and molecular cytogenetic techniques, i.e. G-banding, comparative genomic hybridization (CGH) and spectral karyotyping (SKY) in order to identify chromosomal aberrations. Chromosomal breakpoints involved in chromosomal changes are characterized via positional cloning using fluorescence in situ hybridization (FISH) with breakpoint-spanning YAC and BAC clones. Candidate genes are identified via sequence database blast search and the gene expression patterns are analyzed by RT-PCR and Northern blot analyses. In order to search for predictive markers for radioresistance in tumors, CGH analysis will be performed with radiosensitive and radioresistant cohorts of a particular tumor entity (i.e. head and neck squamous cell carcinomas). Mapping of chromosomal regions distinctive for radioresistant tumors are performed using FISH analysis with YAC and BAC clones.