C-banding stains:
C-banding procedure uses barium hydroxide denaturation followed by Giemsa staining to highlight specific chromosome regions. After alkaline treatment, highly repetitive AT-rich satellite DNA located in constitutive heterochromatin preferentially retains stain, producing dark blocks especially at centromeres, pericentromeric regions of chromosomes 1,9,16 and long arm of Y chromosome. Euchromatin extracted during procedure appears pale. Dark C-blocks correspond to regions enriched in H3K9 trimethylation and HP1 that stay condensed. Technique aids identification of centric polymorphisms, dicentric chromosomes and heterochromatism variations, distinguishing from G-bands and R-bands marking other features.
Ref: Gardner and Sutherland, Chromosome Abnormalities: C-banding Stains Constitutive Heterochromatin; NCBI Bookshelf Clinical Cytogenetics