At a follow-up clinic visit at 6 yr old, considering the relatively high mosaicism rate of up to 30% reported (Krawczynska et al. 2019) in the clinically diagnosed mutation-negative CdLS patients, a CdLS gene panel (sequencing and CNV analysis), including ANKRD11, HDAC8, KMT2A, NIPBL, RAD21, SMC1A, and SMC3, on buccal swab specimens was ordered and identified a novel hemizygous variant, c.793_795delGAG, p.Glu265del, in SMC1A in 60% of sequencing reads (33 of 56 reads), finally provided a definitive molecular diagnosis (Table 2). This evidence concerns the gene SMC3 and Cornelia de Lange syndrome.