Coding sequence cartridges for aminoglycoside phosphotransferase (APT) were isolated from bacterial transposon Tn903. When incorporated into a heterologous gene construction utilising the PGK1 promoter and terminator, the heterologous APT gene provided a G418-resistance determinant that functioned efficiently as a dominant marker for yeast in both multiple- and single-copy. Transformant colonies on selective medium appeared rapidly, within 36-48 h, and growth rate of the transformed cells was normal. A simple and highly sensitive radiolabelling assay for APT enzyme activity was developed for use with crude cell protein extracts. Enzyme activity units were equated to the amount of APT protein present in the cells, and the APT protein was shown to be stable in yeast. Heterologous APT expression was 130-fold reduced compared with homologous PGK1. This resulted from an estimated two-fold decrease in mRNA level and a 65-fold decrease in translation efficiency. The latter was unaffected by AUG sequence context change, but corresponded with a high frequency of minor codons in the APT-coding sequence. APT can be used as a semi-quantitative reporter of gene expression, whose useful features are in vivo detection via the G418-resistance phenotype and powerful cell-free assay.