ACP5 and phenylketonuria: Numerous large tartrate-resistant acid phosphatase (TRAP) positive and multinucleated OCs were identified in the unstimulated PBMC cultures from PKU patients (OC average number/well 149±80; Fig. 1A), whereas few OCs appeared in the unstimulated PBMC cultures from healthy controls (OC average number/well 91±51; Fig. 1B).