Coffin-Siris syndrome is a multiple anomaly/mental retardation syndrome characterized by "coarse" facial appearance, hypoplastic or absent nails on the fifth digits, generalized hirsutism with sparse scalp hair, hypotonia, and developmental delay. Due to several reports of affected sibs with or without a mildly affected parent, both autosomal recessive and autosomal dominant inheritance have been suggested. All previous patients with well-documented Coffin-Siris syndrome are chromosomally normal, and the gene has not been mapped. We report on an infant with typical findings of Coffin-Siris syndrome who also has a de novo apparently balanced translocation of chromosomes 1 and 7, karyotype 46,XY,t(1;7)(q21.3;q34). The parental chromosomes are normal and none of the relatives have signs of Coffin-Siris syndrome. The breakpoints 1q21.3 and 7q34 are suggested as possible locations for a Coffin-Siris gene.