4q35 probe reagent is used to detect chromosomal abnormalities at the 4q35 locus, the most telomeric region of chromosome 4. This region is clinically critical because it contains the D4Z4 macrosatellite repetition, whose contraction is the main cause of Facioscapulohumeral Muscular Dystrophy (FSHD) and is used in broader subtelomere screening panels.