Between light and dark, the chimera comes out.

Background: Chimerism, especially in the absence of sexual ambiguity, is extremely rare in humans. We report the case of a 6-year-old boy whose skin pigmentary abnormalities revealed chimerism.

Observations: The boy had no remarkable previous medical history, and he had normal intelligence and development. On examination, we found a disorder of the skin pigmentation that was difficult to categorize; there was a lighter-appearing skin patch in the median frontal area and also on one-half of the abdominal area, with a sharp midline demarcation. He also had 2 lighter Blaschko-linear bands on the lower extremities and an indefinable mixture of lighter and darker skin on the back and the lateral part of the trunk. It was not possible to ascertain by means of clinical examination of the patient, his parents, and his brother which of the 2 shades was his normal skin color. Because this pattern of pigmentation might be related to mosaicism, we determined his karyotype. We found that his lymphocytes had a normal number of chromosomes, half of them being either 46,XX or 46,XY. In contrast, his fibroblasts were exclusively XY. The chimerism was confirmed by the analysis of the red blood cell antigens, which revealed the presence of 2 different populations. The characterization of the HLA haplotypes of the lymphocytes showed that the boy inherited 2 HLA haplotypes from his mother but only 1 from his father. Interestingly, the ratio of XX to XY cells was expanded in the T-cell subset compared with other peripheral blood mononuclear cell populations.

Conclusions: This is an exceptional case of human chimerism revealed by abnormal skin pigmentation. This boy displayed 2 normal shades of skin color, which we suggest be termed cutis bicolor, as a result of 2 different genetic backgrounds. He also had immune chimerism, which challenges our current comprehension of antigen presentation and tolerance.