Isolation of an abundantly expressed sequence from the human X chromosome by differential screening

A cDNA library was constructed from poly(A)+ RNA derived from MP2H4, a mouse-human somatic cell hybrid, containing as its only human contribution an X-6 translocation chromosome. This library was screened with [32P] c-DNA derived from MP2H4 and counterscreened with a phenotypically similar mouse cell line. From a screen of 4000 recombinants, seven clones were isolated which hybridized more strongly with cDNA derived from the mouse-human hybrid than with the mouse only cell line. Southern blot analysis showed that four of the seven clones originate from the human genome, three of these contain repeat sequences, and one, SCR10, is devoid of repeats. SCR10 identifies a 1-kb mRNA transcribed from the human X chromosome mapping to the region Xq13-q13.3 or Xq21.3-q22 and is an abundantly and ubiquitously expressed gene. A near, or full-length clone of SCR10, SCAR, was isolated and sequenced; the conceptional translation of this sequence encodes a basic protein of 27.5 kd. Sequences homologous to SCAR were detected in primates, rodents, avians, and Xenopus.