Flow cytometry (O’Gorman et al., 1997) confirmed hyper IgM-syndrome confirmed by the lack of expression of CD40L, while parasitology exams, performed on stool by Ziehl–Neelsen and Merifluor® staining (Del Chierico et al., 2011; Del Chierico et al., 2018), confirmed the ongoing infection by Cryptosporidium infection (Figures 1B, C). The gene discussed is CD40LG; the disease is cryptosporidiosis.