Reduced HLA class I antigen expression on CD34⁺ CML cells following acid stripping. Cryopreserved bone marrow cells obtained from CML patient 2 were thawed and CD34⁺ cells were isolated and cultured in 1% BSA (solid line) or acid-stripped at pH 3.0 (bold solid line). An IgG1 isotype was used as a negative control (dashed line).