PTCH1 and medulloblastoma: For example, a rare population of CD15-positive cells identified in human medulloblastomas (Read et al., 2009) and from a Ptch1+/− medulloblastoma mouse model (Read et al., 2009; Ward et al., 2009) also express ATOH1 (also known as MATH1), and so resemble granule neuron precursors rather than stem cells.