A male infant was observed with a complex of un usual clinical abnormalities. Chromosomal studies revealed that the child was mosaic for a chromosomal fragment from the long arms of a number 9 chromosome. The majority of cells contained 46 chromosomes with a partially deleted number 9 chromosome and a chromosomal fragment. From these data and others repor ted, it would appear that there is a susceptibility for breakage of the long arms of the number 9 chromosome at the junction of the heterochromatin and euchromatin area closest to the centromere. The lymphocytes from this child are capable of transformation into long-term lymphocyte cultures, and after many months in culture the chromosomal fragment has replicated in the cultured cells even though it contained no detectable centromere.