The P elements in otu¹⁸, otu¹⁹ and otu²¹ are inserted at exactly the same base position, between 14 and 15bp upstream of the first major transcription start site, designated as +1 in Comer et al., Gene 118: 171-179. The target site duplication GGCTAGAT, located between -14 and -7 is found at both ends of the insertions. Southern analysis revealed the otu¹⁸ P insert to be apparently full length and in the opposite transcriptional orientation to the otu gene.

P-element is of molecular size 2.9 kb (Kalfayan, Walsh, Ousley and Nishiharu, unpubl.).