A heterozygous missense mutation has been described in patients with idiopathic CD4 lymphocytopenia, an immunodeficiency disorder, that disrupts the Unc119-Lck interaction to stimulate Lck catalytic activity by the TCR.[45] Patient have < 300 CD4 T cells/mL (20% of total T cells) and a history of recurrent sinusitis/otitis media, widespread onychomycosis, fungal dermatitis, oral lesions, and obstructive bronchitis with pneumonia. Here, CD4 is linked to susceptibility to pneumonia measurement.