FISH was used to confirm the presence or absence of TWSG1 and TGIF, a gene known to be associated with HPE, for 8 patients with holoprosencephaly (HPE) and deletions of 18p. Red arrow indicates TWSG1 present, red arrowhead indicates TWSG1 absent, green arrow indicates TGIF present, and green arrowhead indicates TGIF absent. Patient identification indicated by numbers. The HPE types of patients 1, 3, 4, 6, and 8 are severe or likely severe. A sample from patient 10 was not available for testing; as a surrogate, the patient’s clinically normal brother with 46,XY,t(6;18)(p24.1;p11.21) was tested; FISH showed ish t(6;18)(RP11-22C6+, RP11-66J9+; RP11-22C6−, RP11-66J9−) (not shown).