Fluorescence diagnosis after application of 5-aminolevulinic acid (ALA) detects red-fluorescing preneoplastic and neoplastic lesions using light excitation. The principle of the method is the relative tumor-selective accumulation of the metabolite protoporphyrin IX (PPIX), which is built intracellularly out of exogenously applied ALA. The early detection of tumors and especially preneoplasias is an ideal prerequisite for genetic analysis of these lesions. With this approach, methods such as fluorescence in situ hybridization and loss of heterozygosity analysis for deletion mapping as well as gene sequencing data could be compared. New data are presented on deletions, numeric chromosomal aberrations, and oligoclonality of tumors found in about 30% of cases. The phenomenon of tumor-selective fluorescence was further investigated by parallel biochemical analysis, which showed marked differences in heme metabolism. The analysis of gene and protein expression may aid in identifying tumor-specific molecules associated with heme metabolism.