FISH detection of Y-chromosome in donor AM from a recipient of sex-mismatched BMT (donor, male; recipient, female; on day 48). In situ hybridization was performed using Y-chromosome–specific cocktail probe (DYZ1/DYZ3) as described in Materials and Methods. Y chromosome is stained as a bright yellow dot in each nucleus of male AM. In the sex-mismatched BMT (1; the arrow indicates donor cell), small AM have Y chromosome (donor origin) but not with large AM (recipient origin).