PHtD_u1 is a natively unstructured region on Pneumococcal histidine triad proteins of higher eukaryotes lying between the first two Strep_his_triad domains so far identified, Pfam:PF04270. The function is not known but it does not carry the characteristic histidine triad.

1: New insights into histidine triad proteins: solution structure of a Streptococcus pneumoniae PhtD domain and zinc transfer to AdcAII. Bersch B, Bougault C, Roux L, Favier A, Vernet T, Durmort C; PLoS One. 2013;8:e81168. PMID:24312273