The case of a premature underdeveloped male baby showing an asymmetrical face and minor congenital malformations is presented. The baby expired on the 6th day of life. Chromosome studies by light and fluorescence microscopy showed a translocation between the distal two‐thirds of the No. 14 chromosome and the Y chromosome (46, X, t(Yp+; 14q –)). The karyotypes of the parents were normal. The father's “Y body” was morphologically similar to that of the propositus. The cytogenetic findings are relevant in the light of the present hypothesis that the male sex determining factor is to be found on the short arm of the Y chromosome. The translocation chromosome contained the whole long arm of the Y chromosome, its centromere and the centro‐meric portion of its short arm. This suggests that the male determining factor is located on the centromeric portion of the short arm of the Y chromosome.

Identification of D group chromosomes was done by quinacrine mustard staining and fluorescence microscopy, as well as by 3H–thymidine labeling and autoradiography. No special instruments were used for the fluorescence microscopy studies. Chromosome 13 was recognized easily by fluorescence microscopy by its typical fluorescence pattern in the lower two–thirds of the long arm, the region of the late labeling in autoradiography. Chromosomes 14 and 15 could be recognized by fluorescence microscopy in most cases. If differentiation is not quite definite, 3H–thymidine labeling and autoradiography should be used to support the findings.