A tandem array containing 12 copies of a mutated histone gene repeat unit; in each repeat, the entire 298bp sequence between the initiation codons of the His3 and His4 genes has been replaced with the 226bp sequence from between the initiation codons of the His2A and His2B genes. In addition, in each repeat a number of synonymous mutations have been introduced into each histone gene (resulting in the addition and removal of restriction enzyme sites) so that the transgenic repeats can be distinguished molecularly from the endogenous copies.

Wild-type His1 gene in the context of a histone gene repeat tandem array. A number of synonymous mutations have been introduced (resulting in the addition and removal of restriction enzyme sites) so that the transgenic gene can be distinguished molecularly from the endogenous gene copies.

Wild-type His2A gene in the context of a histone gene repeat tandem array. A number of synonymous mutations have been introduced (resulting in the addition and removal of restriction enzyme sites) so that the transgenic gene can be distinguished molecularly from the endogenous gene copies.

Wild-type His2B gene in the context of a histone gene repeat tandem array. A number of synonymous mutations have been introduced (resulting in the addition of restriction enzyme sites) so that the transgenic gene can be distinguished molecularly from the endogenous gene copies.

His3 gene in the context of a histone gene repeat tandem array in which the entire 298bp sequence between the initiation codons of the His3 and His4 genes has been replaced with the 226bp sequence from between the initiation codons of the His2A and His2B genes. In addition, a number of synonymous mutations have been introduced into the His3 transcription unit (resulting in the addition and removal of restriction enzyme sites) so that the transgenic gene can be distinguished molecularly from the endogenous gene copies.

His4 gene in the context of a histone gene repeat tandem array in which the entire 298bp sequence between the initiation codons of the His3 and His4 genes has been replaced with the 226bp sequence from between the initiation codons of the His2A and His2B genes. In addition, a number of synonymous mutations have been introduced into the His4 transcription unit (resulting in the addition and removal of restriction enzyme sites) so that the transgenic gene can be distinguished molecularly from the endogenous gene copies.