(A) FISH with YAC 746H8 (red fluorescence) containing the PTEN/MMAC1 gene and YAC 790H10 (green fluorescence) containing the MXI1 gene on the cell line Karpas 422. The interphase nuclei display two green signals indicating the two copies of the MXI1 gene but only one red signal indicating loss of one PTEN/MMAC1 allele. Some signals are out of focus. (B) Genomic sequencing exon 3 of PTEN/MMAC1 in a control (top) and Karpas 422 (bottom) revealing a splice acceptor site mutation (A → C) at position −2. (C) RT-PCR analysis of a control (lane 1) and Karpas 422 (lanes 2 and 3) with primers surrounding exon 311 showing a lack of the expected germline 397-bp fragment but a 45-bp shorter PCR product caused by skipping of exon 3. Lane 4, H₂O-control. Lane 5, 100-bp ladder.