Significance of phenotypic and chromosomal abnormalities in X-linked mental retardation (Martin-Bell or Renpenning syndrome).

With the exception of macro-orchidism, three families with X-linked mental retardation showed diagnostic concordance of clinical features among the affected males. Since macro-orchidism was a variable feature among the otherwise identically affected males in one family, we question the existence of a separate entity of X-linked mental retardation characterized only be testicular enlargement. The X chromosome marker of Lubs was expressed, under the culture conditions of Sutherland, in lymphocytes of the affected males of two families, one with and the other without megalotestes. Two affected members of the third family, with megalotestes, did not show the marker. Telomeric structural changes similar to the mar(X) (qter) formation were found on certain autosomes, notably, chromosome 6 in some of the affected males, potential and obligate carrier females, and in both related and unrelated normal males. These autosomal markers appear to represent a nonspecific response to either in vivo or in vitro folate deficiency. Caution against premature introduction of this test for prenatal diagnosis, in the face of current ignorance regarding diagnostic specificity, is urged.