The holandric gene is Y-chromosome linked. The mammalian Y chromosome appears largely heterochromatic under the light microscope and it carries few genes. The H-Y antigen gene has been assigned to the proximal region of the long arm of Y and the testis-determining factor, formerly called TDF, now SRF, is proximal to the centromere in the same arm in humans. The long arm also contains the pseudoautosomal region (PAS); this DNA sequence Yp (SMCY) is homologous to a X-chromosomal tract, Xp (SCX), the region where X and Y crossing over can occur. The gene for surface antigen MIC2Y was assigned to the euchromatic region Ypter - q1 of the Y chromosome. The homolog was assigned to a X-chromosomal band between Xp22.3 and Xpter.