Laird等提出在Xq27至少具有4个顺式活动基因,这些基因在适当的条件下对延迟Xq27DNA复制有逐渐增加的趋势。他们提出迟复制的DNA发生在特定的染色体部位,这一特定部位较有可能就是体外培养的脆性部位。当这一部位在失活的X染色体上时,假定的突变(第三)基因在母体减数分裂时关闭顺式Xq27的活性,形成印迹(第四)基因。 Laird et al. Proposed that there be at least four cis-acting genes in Xq27, and that these genes have a tendency to gradually increase in delayed Xq27 DNA replication under appropriate conditions. They propose that late-copy DNA occurs at a specific chromosomal location and that this particular site is more likely to be a fragile site of in vitro culture. When this site is on the inactivated X chromosome, the putative mutation (third) gene turns off cis-Xq27 activity at maternal meiosis, forming a blot (fourth) gene.